Depends on how many you are selling. I would suggest going to a local coin shop. There are also a couple of online companies that buy them. I have seen others mention Provident and JM Bullion.
Yes, 1986 & 1987 $50 Gold Proofs are listed for sale on eBay. Prices starting at about $1300 & up. I guess it depends on what you paid for them originally. And then what you would net $ after eBay & PayPal fees.
Here are some 'buy' prices from a company that I often hear being praised by others, APMEX. https://www.apmex.com/apmextop40 (Click on 'top products') Here is a place to find other companies, as well as know current prices: https://comparegoldprices.com