Rule 1. If it's not your disc, it's not your disc.
Rule 2. If it's marked as someone else's disc, it's not your disc.
Rule 3. If all reasonable attempts to return the disc that is not your disc and marked as someone else's disc which you found have failed, it is now your disc.
The clown snagging your disc from the pond violated/skipped Rules 1 and 2. He then attempted to sell it for monetary gain which could be considered a criminal act because the property was essentially lost/stolen. Granted there is potential for he said/she said yet at the very least he should be labeled a crook and reported to eBay as well as blackballed in disc golf community.
The people that have been caught selling lost/found discs typically do get shunned/blackballed in the disc golf community. But at the same time, they often aren't the types that play events or show up at club functions anyway, so they probably don't care about being shunned or blackballed.
Bottom line, if you don't want to see your disc end up on eBay or craigslist, don't leave it at the park. If you decide to give up your search or decide not to dive into a pond after your disc, of course you would hope the person that finds it would return it to you. But you should also be prepared to accept that the person that finds it might not return it to you.