Selling: I typically come up with a fair value for the pin, then subtract 5-10%. Easy and quick is better than a few extra bucks IMO

Buying: Living in a pin-remote area, I typically stick to NIB (although I'd never pass on a good local DMD deal). NIB means quick hassle free shipping (STI to me is always at least 3 weeks and $500), no payment negotiations, and reputable sellers. I know NIB only is frowned upon by some in the hobby, but if you look at my collection history, it took me fixing up/selling almost 20 older games to get to this point. Buy low, fix up, sell. Buy mid tier, fix up, sell, etc...