It sounds like you want your users to maintain bitcoin balances on your server and then they'll use these balances to pay for things.

Unless you are very very experienced in computer security and system design, I'd advise against this.  It's only a matter of time before you'll be hacked.

I really don't know why you'd want to do this anyway.  If you sold items  in fiat currency, you wouldn't require users to store fiat currency on your site, would you?  They'd just use credit cards or something.

Please explain further.