I would assume that most dealers buy larger lots or collections to break down for retail sale. Buying individual items for resale would in most cases be a losing proposition, unless you have a client waiting in the wings and you know what they will pay for it.
It takes a while to be able to gauge approximate wholesale and retail values of collections, i.e., "what can I make on this once I break it down?" This is even more difficult buying online when you only see representative pictures, or blurry, too small, etc.
Most of my large lots and collections have been purchased at my local shop or in person from other collectors or at shows, where close examination is possible.
When I buy a bulk collection of U.S. revenues (I'll leave my worldwide classics dumpster-diving out of the mix for now, since it's not done seriously and is more of a pleasant diversion), I am looking at several factors:
1. The comparative value/cost of the "keepers", the items that I want to keep for my personal collection, versus the remainder.
2. Of the remainder, how much of it is suitable for single-item lotting vs. sets or mixture fodder. I rarely list anything for less than $5 on
ebay; it's not worth the hassle (and that threshold is much lower than many others I know). For APS stamp store submission, however, I'll go as low as $2-3 items since I'm not the one that has to deal with payment or fulfilling the orders.
3. Are there groups of stamps I can sell to my plating, variety, and/or cancel collecting acquaintances that will help me recoup a large percentage of my outlay quickly.
4. Rather than taking the time and labor of breaking the collection down, am I better off pulling a few key items to keep or sell individually and then "flip" the collection relatively intact? Potentially not as much profit, but less work involved and a shorter period that money is tied up in the original purchase.
There are no hard and fast answers.