There is or was a market for it. I read on this forum or in Treasure Magazine that out West they find it with there detectors in vessels and they were getting quite a good return on it. Sorry I don't know where they were selling it. Also panners find it in streams, do a search in metal detecting forum or ask there, maybe someone could help.
This is an old one from many years back. A long time ago I heard motorcycle repair shops use to buy mercury. They somehow used it to tune carburetuers, perhaps Harleys.