A recent phone conversation from a fellow "dealer" went something like this. This gentleman was looking for full mint sheets of 1920-era stamps for his customers. He inquired if I had any. I had one sheet that met his needs. He suggested that if I find more sheets that I contact him. His example went something like this. "If I can sell a sheet for $200, I make $100 and you make $100." What a generous offer. If a sheet is worth $200 to someone else, does he really think I'm going to get it for nothing? On a side note, if you have any older mint sheets you would like to give to me, please let me know ASAP. I need to return this gentleman's phone call.
There are "dealers" (and I use the term loosely) out there who don't advertise. They don't belong to the APS and/or ASDA. They go around filling want lists for collectors by cannibalizing the stock of other dealers and getting them to low-ball their price. These dealers are out there for one purpose and one purpose only, to make a quick buck. They are here today and gone tomorrow for the next money making scheme.
I don't mind doing business with other established dealers as long as they have a Tax ID. However, I won't cut my prices or cut items out of my stock (thereby limiting your selection) for these fly-by-night and get-rich-quick "dealers". No thank you.
Most legitimate dealers are members of professional organizations. They usually advertise in one or more major philatelic publications. They build relationships with customers. They want to make a profit, but not at someone else's expense. They make a fair profit over advertising, insurance, printing, and a host of other business expenses.
Whether you deal with me or any of the other reputable dealers out there, I encourage you to look for dealers who belong to the major organizations and/or who advertise regularly. They deserve your support. They help build the hobby.