That could be from a charge back, bounced check, or because the cash was determined to be counterfeit when deposited.
A big problem is the costs involved may be 10% at scale, but are certainly more than ten cents per transaction, so you need to get a deposit, but I'm not buying $50 worth of flooz to pay someone $1, because I don't know when flooz is going to stop working. Anyway, it's not that hard for someone to run a PayPal shop, as long as the product fits PayPal's tos and doesn't look suspicious.