Ooh, I'm not sure!
I don't see why there can't be? If Gringotts just deals with money, there must be a way for wizards who want to deal in shares to make some cash. I'm not sure they'd be regulated by the bank in this instance. I think it's plausible.
I'd be inclined to say go for it (providing you give a good explanation of how your character would cheat the system and depending on how much money they have, you may need a rich EFB application) but I'll let someone else give you a definitive answer, I just didn't want to leave you hanging xD
- Sioban