The publishers of catalog prices are not dealers, they are publishers providing "Best" knowledge of what you (James) might expect to pay a dealer for a given stamp in solid used, or unused condition. You, will not get those prices 99% of the time when you (James) sell your stamps as an individual. Buyers do not ordinarily publish price lists...you will get whatever someone offers you on any given day...supply and demand. I hope that correctly summarizes how the market works. Anyone else?
Well, some catalogue publishers, such as Gibbons and Yvert, certainly are dealers. A rough guide to real world value for most stamps is 10% of Gibbons's price - obviously more for scarcer items, much more for rare items, At the bottom end of the market, Gibbons will price a stamp at a minimum level, but they are, in effect, worth next to nothing.
For less-common items, an auction is probably the best way to find out what an item is worth.
You can try to set a fixed price for something, and if it sells in the first minute, to the first looker, you probably didn't ask enough, and if you still have it six months from now, you probably asked too much.
It sounds like we might need just slightly more information about your plans in order to give the best answer...
So you will not sell to dealers -that is interesting . Roman States stamps sell for a lot less that 10% of catalog ,as they say condition of the stamp is everything and the area of Roman States has many different forgeries and reprints.