I've been doing the same, but on a very small scale. Mostly sending a funny episode of some show to a group of friends or occasionally sending a movie to my folks. I don't doubt that if I was mass distributing these files it would attract attention.
Also, wouldn't breaking the hash nullify one of the ostensible advantages of this method (the de-dupe of the stored files)? If the goal is solving global file distribution, making each copy of every originally-identical file unique - and therefore requiring n times the storage - isn't a viable solution.
I don't know about the pictures thing, but the reputation system works pretty damn well. I buy stuff off of ebay regularly and find that especially high profile sellers do everything they can to keep their reputation up, because otherwise they lose potential clients.
You just really do need to ingrain into the community that a good reputation is essential, damnit, and don't buy from someone that doesn't have one. Then the sellers conform to that.
EBay's reputation system is a joke People routinely create fake accounts, farm for reputation by making lots of low-value purchases, then wait a few weeks until you cannot browse the details of their purchase history.
Once that happens, they move from buying or selling recipes for $0.99 to trading laptops, scam a few people, then vanish.
Actually eBay is quite safe to buy if you pay via Paypal. eBay has a policy of 45 day return period during which you can file a case in eBay resolution center under "item significantly not as described" and send the item back to the seller, eBay will then refund the full amount you paid to the seller (price + shipping). However, you have to pay for the return shipping. I believe payments via Paypal on a 3rd party website are also covered by the same policy. This makes it very safe to buy from eBay or through Paypal. I feel more secure buying from eBay then buying from a local retailer.
The reputation system is probably (read likely) gameable, but that doesn't mean it doesn't have any value. Ebay has a very buyer friendly system system that the risk of getting scammed is pretty low if you follow the rules set for Paypal.
How so? I think you can trick people into reviewing before they get their item by prompting feedback with an email, but not everyone falls for that and also it's sort of hard to gather substantial reputation like that (100+)