You’ll find selling “garden variety” used LPs to be a slow, unprofitable process. It gets a little better when you find a few locals who will buy them for a buck right from your store. But, even then, you’ll have a bunch of leftovers for the dumpster. If you want to get into “collector level” used LPs typically desired by collectors, you’ll need to dive into a process of learning what titles are desired and how to grade them for quality, not unlike postage stamp or coin collecting. Now you’re talking a labor of love since the buyers will be picky what they play on their $5,000.00 turntable. A website to look into is Audiogon.