I've sold a lot of stuff on discogs; haven't bought any vinyl.

Look at the quality. It's self-assessed, but you can look at a seller's ratings. You can also ask them questions and ask for a photo.

As for costs: there's no "strategy." Add it up.

you want to have a look at Operations Research[7]...im not an expert however, and could not possibly point you to the specific solution

[7] https://en.wikipedia.org/wiki/Operations_research