Lots of good points in this thread. Thought I would add my comments from the perspective of both buyer and seller (on
ebay, Bidstart, delcampe mainly).
My expertise is in Canadian stamps only, so my comments may not apply to other countries necessarily.
Buyer: Deals are much harder to find on all these sites (from efven a few years ago). I agree that many sellers do overprice and wait to see (Bidstart has low fees, so this is more common than on
ebay since their fees continually rise and you pay to have things sit...unless you use the free 50 listing/month option and only sell auction style)
The bargains are still there, but you have to look much more frequently and at different times. Once you find a good seller, I recommend you keep them on a short list and check back for new items on a regular basis, as KGV mentioned.
Prices: I use the Unitrade Canada Specialized as my main guide. I think the prices are fairly close to market in many areas. You really have to read the caveats and notes in the catalog, not just the prices.
The thing that many novice collectors don't always understand is that quality is crucial in value. A Fine stamp is about 50% less in value (or more) than a Very Fine stamp. Yet many assume that the stamp they have is Very Fine when it is not. Also, very fine classic and rare stamps will often sell far above catalog value because experienced buyers know that they do not come along that often and are willing to pay to add it to their collection.
For easy to get stamps, it just becomes a matter of what is a minimum price to ask in order to make it worth selling. In this, the catalog values are more or less irrelevant and are more of a place holder value.
Seller: I sell mostly fixed price with a basic
ebay store. As this is a hobby, not a business (I sell extras to buy things I need), I want to help buyers find items at a good price that is fair for both. In this I have noticed a decrease in prices buyers are willing to spend compared to a few years ago, but the stamp market (like all others) has its ups and downs. It also depends a lot on what you sell as some stamps are in more demand than others at different times.
As I mentioned, I see many sellers price their items higher than I would (either through ignorance, greed, perhaps they have a clientele that does not shop around, or the seller does all the extras well and their customers are willing to pay for the service - in which case they price according to their own market and new customers may feel the prices are high if that is all they see).
I base my prices on
ebay completed listings that have sold recently, and my own experience (since I sell many unique items - varieties, etc that do not come on the market very often). All buyers can check
ebay completed listings if they want to see the prices that items actually sell at. I would recommend that as a good way to get a feel for the true market.