I was forced to sell the account as the user had defaulted on his loan
Nothing forces you to sell the account. I believe I still have a few accounts that defaulted loans in my possession, and have just accepted it as a loss.
Yes, I believe that it is acceptable for a lender to take an account as collateral, if they believe that the person behind the account is likely to motivated to repay the loan and get their account back. For example, if I were desperate and needed a quick loan, if I put up my account as collateral, I would move heaven and earth to make sure that loan was paid. The only way that I would default on the loan is if I became incapacitated.