From my experience in different NYC record stores (often while browsing through bins in the store I hear such conversation between owner/clerk and person willing to sell records). They ALL will try to rip You off! There's no other way, best case scenario, maybe they will offer You 1/4 of the actual value. Actually, right now I have about 300 records that I no longer need, the ones that I already upgraded to much better pressings, still a lot of them much better/rarer than stuff I see nowadays in any local store. Any time I tried to sell something to the store, I was lowballed to the point it was just insulting. Considering that, I don't like selling through internet, can't stand dealing with all kinds of idiots and crooks, maybe one day I will buy a table at some record fair and try to sell it there, not even for the profit, but at least to get back whatever I spent on them. Until then, I'd rather give some of them away to friends or even dump some of them, but I will never bring any of this stuff to record store - to make any of this crooks rich .
Do you use Discogs to catalog your stuff? Could just make a folder marked "For Sale" and share it with US. We would never low-ball you if there are titles we want... Right everyone?
Factor in the expense of buying mailers, standing on line at the post office -- in my case having to get on a bus or streetcar to GET to a post office because the one in our neighborhood was condemned and they can't be arsed to repair or replace it -- mailing costs...there's a reason my culls just go out on the stoop for whoever wants them.