Historically I have excoriated Apfelbaum for their looseness in describing items (to put it mildly) and their treatment of buyers with too many returns as a result of Apfelbaum's misdescriptions.
That said, in the interest of fairness, if I'm going to go out of my way to rip them, then I should also post if I have a good experience... which amazingly enough, I just did.
So some may be asking "Why do you buy from and support a seller/dealer you know has ethical issues? Doesn't that send the wrong message?" Well, to be brutally honest, if buying from a less-than-reputable dealer will garner me material I want at a price I like, I'll make a deal with the devil himself. The ends justify the means.
Also, my not buying from them is unlikely to have any effect on a dealer of that size.
No, I've purchased from Apfelbaum, Aldrich, Anthony's, NYStamps, Bill Langs, and any number of other dealers that receive large numbers of complaints when it has suited my needs... after all, it *IS* all about me.

Heck, if I didn't purchase from any sellers that I had moral, ethical, political, or philosophical differences with, I wouldn't be buying much of anything, as I'm disagreeable with just about everyone.

In my experience, the best way to get good deals from these types of large dealers is on group lots/collections that contain specialized material, if you can find items that they missed or misidentified... and then pounce. Use your knowledge to good advantage.
With Apfelbaum, ignore the prices on their online storefront. Sign up for their email list. They send out coupon codes that change almost daily. They usually range between 20% and 35% off, but occasionally they will go as high as 40% and last week 45% off. (That should tell you how overpriced the nondisounted prices are.)
Recently they added a bunch of RN bulk lots (revenue stamped paper) to their inventory, an area where my collection is woefully deficient and I have been wanting to add representative examples to my collection for quite some time, but not at retail prices.
[Please note: RNs are an area where I am an absolute novice. Take my comments with several grains of salt until/unless more expert RN collectors chime in.]
Apfelbaum separated them by type (all the RN-B in one lot, RN-K, RN-P, RN-X, etc.) and had multiple scans for each lot showing representative items or "better items" (To err on the side of caution, I always assume that items shown in lots like these are the BEST items in the lot, and those not shown have excessive duplication of common types and designs).
So I went through the descriptions and images meticulously and checked their itemization versus (1) the Scott Specialized Catalogue, and (2) Eric Jackson's inventory. Eric is a specialized dealer and prices his inventory accordingly; he is on the high side of retail, typically above Scott. However, he carries very scarce items, and more importantly here, a wide coverage of Scott numbers. He has a loyal clientele and rightfully so. I've purchased a large amount of material from him over the years. Richard Friedbarg as well, but Richard's website is sadly stuck in 1998 or so and isn't as good a reference as Eric's website.
An additional aspect that you have to factor into RN pricing that Scott does NOT help with, is the nonphilatelic components of the value of a stock certificate, bond, check, etc.: the companies in question, the signatories, and aesthetics (vignettes, usage of color, etc.). These are aspects that many philatelic dealers ignore that Eric does not. Even if Eric's retail price is above normal, by comparing against other items of the same type that he stocks, I can gauge which items are better than others.
Also, since online image quality is mediocre at best, gauging color variances (which can be ever so slight with RNs) and verifying that a claimed better color really is what Apfelbaum lotted it as, can be difficult. This is another area where Eric's website comes in handy. Rather than relying on color in images, I can compare the company and design of the document against how Eric has it identified as an indicator of accuracy. Granted, this method fails if a company had documents with both RN Color X and RN Color Y and the colors are similar, but I find those situations to be few and far between. Usually the company and check/bond design are good differentiators of Scott number.
Another good reference for market value and scarcity for Railroad stocks and bonds, and Coal company certificates, is
http://coxrail.com. I always check that when evaluating railroad material as the philatelic component of a document's market value might be much smaller than other aspects of the document.
So after doing my research, I determined that with a 40% or 45% off coupon, on average I would net material at roughly 30-35% of Scott, which isn't bad for a bulk lot *IF* the material is (1) as advertised, and (2) a decent variety of types, colors, vignettes, etc. without excessive duplication. I also calculated a "per-piece" cost for the material and gauged whether I would be able to sell the extras on
ebay to help recoup the expense... or would I be buried in the lot?
For some lots, that per-piece calculation was enough to buy the lot outright. For example, RN-X types where nice examples sell for $3-15 on
ebay, my cost was right at a dollar per piece... worth the gamble. RN-U types, mostly railroad and mining at roughly $13 each. Easy sellers at $20-40 each depending on company. You get the idea. The goal is to be able to pull keepers for my personal collection and have a good chance of flipping the overflow without loss.
On other lots, what tipped it over the edge was finding items in the scans that were not correctly identified. For example, the RN-B lot (which had some lovely material scanned) showed at least one RN-B1 ($5.00) that was actually an RN-P5 ($50.00). The RN-V4 lot (cat $175 mint) had one piece that was actually an RN-V6 (cat $450 mint). Things like that.
There were a few lots where they hadn't uploaded the promised images, or there was ambiguity in the description and/or stated catalog values. When I inquired, communication was fast and I usually had images emailed to me within 24 hours. Excellent communication and responsiveness. Even when they couldn't get to my request for images or checking a lot right away, they at least responded right away to let me know there would be a slight delay.
So I placed my orders (3 separate orders over approximately a week), and I have to say that Apfelbaum's order fulfillment is about the fastest I've seen from a commercial stamp dealer. In each case, the order was packed and shipped within hours of my placing the order, and while I didn't get any order shipment email, all 3 orders were in my PO Box in less than 48 hours from placing the order (they are in PA and I'm in IL).
I held my breath as I opened and evaluated each lot. Overall I am very pleased. I'll be keeping them all; no returns. Much less duplication than I expected, and some extremely lovely aesthetic pieces; great variety. This was either part of a dealer stock or an advanced collector's overflow material, as in addition to the variety, the condition on average is excellent, unlike some of the ratty lots you sometimes see on
ebay.
There was one lot that fell significantly short of the promised count, but the quality was much better than I expected (and I found a few mis-IDs in my favor) so I'm content with it. It happened to be the lot with the cheapest per-piece cost, so the impact of the shortfall was not dramatic.
If I had to grade the overall experience I would give it a solid B+/A-. Surprising really, given my previous (and other people's) experiences with Apfelbaum.
I still don't recommend them if you are a novice or don't have the resources to effectively research what you are buying, i.e., if you are relying on THEIR expertise rather than your own for identification, but if you have your ducks in a row it can work out.
Images of material forthcoming, but it will take me months to process all of it.