Auction them in classifieds and you will soon find out.
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Roxy is correct. If you do an auction you can set a reserve for the lowest price you will take for your records. If the highest bid does not meet your reserve there is no winner. You can also do this on ebay or discogs or other sites for a wider set of potential buyers.
You can also list them in a classified for the price you would like to receive and see what kind of offers you get. You then have the ability to lower the price or let the ad expire if you're not getting offers at your preferred price.
If you want to get top dollar you may have to go through this process more than once to find that buyer willing to pay top dollar.