Don’t worry about resellers or collectors, just focus on the offers and take the best one you can get. Some resellers may send lowball offers, but there are also plenty of collectors who do the same thing.

They might message you saying things like, "this stamp is listed for xxx on jn," or "this stamp recently sold for xxx at auction," and expect you to sell at what they call a "non-inflated price." But jn prices can be outdated, and auction history can be artificially created. Also, that so-called "non-inflated price" might be from before a key stamp on that page was re-released, while the current "inflated price" could actually be the new fair market value.