If you don’t want the fish to breed then limit the tank to a single sex.
Regarding selling fish, common live bearers are hard to sell (in my opinion). I find the fish typically reproduce faster than I can get rid of them. I don’t think Petco/Petsmart take fish; they definitely don’t buy them from local hobbyist. You can check with your local fish store(s) to see if they would take them for store credit (or for free); expect very low value for the exchange if one is offered. There are swap meets where people sell fish; I don’t recall seeing anyone selling mollies, so there *might* be a local market for it. Occasionally, I see postings for free fish on the forum but don’t recall if they made it to a new home. The AAAA typically has auctions where I think the mollies would sell; no telling on price.