As far as i can tell there are 3 major problems

1) customers being able to cancel their deposit payment through their payment processor

2) legal nightmare of who owns what when. As far as i recall its highly problematic to hold funds for somebody

3) legality and taxes

I think it is something that would be best solved with crypto (no this time for real) and having the default be that the depositor gets his money back. But that leaves the entry hurdle (and cost in and out of what ever coin) as well as fluctuations in price.