When it comes time to sell some stamps, the more prepared you are, the better your chances of coming to a successful conclusion to a sale.
Be prepared to describe your stamps. A question like, "How much are FDCs worth?" won't generate an offer from me. I may give you some general guidelines of what things are worth with cachets, addressed or not, and so forth. If you said, "I have a used set of Columbians, F-VF condition and the 50’ has a small thin," that's pretty good! A general description for starters is OK. Something like, "I have an album of US stamps. The stamps before 1920 are all used and it's 80% filled. After 1920, they're all mint and never hinged with no blank spaces. The stamps don't look to have any faults." That gives me a relative size, range and condition of the material.
When you have stamps to sell, dealers want to know
how much stuff you have (shoebox, an album, a houseful)
a range of years (or range of catalog numbers) that it covers (my grandfather saved stamps during the 1950s)
whether it's US only, worldwide, topicals, specific countries, etc.
the type of material: mint stamps, used, plate blocks, covers, etc.
what your asking price is
Let me give you some advice about an asking price. This is always a hot topic.
Whether it's realistic or not, everyone has some idea of an asking price. If you brought an album to me with US stamps in it and I asked, "Will you take a million bucks for it?, you'd have the check in the bank before the ink was dry. If I asked, "Would you take a dollar for it?", you would think I'm a crook. Now I know the figure you have in mind is somewhere between a dollar and a million dollars.
Sellers are skeptical when a dealer says, "What's your asking price?" If the material is worth a lot more than the price they have in mind, the dealer will "steal" it for their lower asking price. Honestly, there are probably a few bad apples out there that work this way. This is especially true when a family member has inherited some stamps and they have no idea of their value. Overall, I think such occurrences are rare.
You can protect yourself in a couple of ways.
First of all, go to someone that is an APS or ASDA dealer member. They follow a strict code of ethics. It doesn't guarantee that you won't still find a shark. You have a much better chance of getting a fair offer from an APS or ASDA dealer member.
Second, get a couple of offers. If several different dealers all offer about $100 for your stamps, then it's probably worth about $100. You dont have to take the first offer you get. Be up front with the dealer and tell them that you want to get a couple of offers before you decide to sell.
Third, do a little investigation of your own. Look at price lists and auction catalogs. Check around on the Internet. Look for items that are similar to what you have to offer. What do they sell for? Unless you have a highly specialized item for sale, you're probably going to find something for sale somewhere that is similar to yours. Make sure you take condition and centering into consideration.
Fourth, organize your material, especially better items. If I buy a shoebox of used common looking stamps, my offer is going to figure that every stamp in there is common and cheap. I will take a quick look for overall condition and to see if any better items jump out at me. If I spot any better items, I'll take a closer look. If not, I'm not going to examine every stamp in detail because 99.99% of the time a mixture of common looking stamps has no better items in it. I'll buy it as cheap material and that's how I'll sell it. If you're not willing to take time and find that copy of Scott #C3a in the middle of the shoebox, then I'm going to assume one isn't in there. If you have any better material, set it aside or somehow bring it to my attention. If you're only selling a single stamp or a couple of stamps, this isn't a big deal. But in large collections or accumulations, I will look for better items. If I can't find them easily or you don't tell me where they are, I may miss them among the hordes of common stamps.
There is a reason why dealers inquire about an asking price up front. If their offer price and your asking price are significantly different, you're probably not going to conclude a deal at all. If I think a collection is worth $100 and you think it's worth $1000, we're probably not going to do business. There is no sense in bargaining over price and wasting time for both of us. If I think it's worth $100 and you were thinking $150, then there may be common ground to negotiate from.
When I give a quote, I stick to it. My first offer is always my fair and best offer. I don't subscribe to the philosophy of "offer low and they'll ask high and we'll meet in the middle". I don't mind if someone says, "Did you see this stamp over here which is worth a lot more money?" It's possible that I overlooked something. I don't mind sellers who point something out. Likewise, I try to point things out to sellers (both good and bad) as I go along so that they know what my thoughts are. I say things like, "These gold stamp FDCs don't sell well" or "These stamps are damaged which ruins some of the value" or "These are better plate blocks."
Here is a technique I've used a couple of times with some success. When I'm reviewing material, I'll write down an offer on a piece of paper and turn it over so that the seller can't see my offer. I tell them that I wrote down my offer. Now they know that I can't change it. Then I'll ask them for their asking price. They are now much more likely to give it to me because they know I can't change my offer price.
This has worked successfully for me several times. I do spend time looking something over and maybe I won't buy it. That happens anyway. But I think sellers feel more at ease that I'm not going to steal the collection if they give me a low asking price.
If we're far apart in our figures, we can usually conclude business very quickly. If we're close, then the seller already knows what I'm thinking because I pointed out good and bad things as I reviewed the material. I won't go higher unless I made a mistake or missed something. The only question at this point is, will the seller come down to my offer price? I've never had someone reject my offer when my offer price was higher than their asking price.