We had a problem with a member who it turned out ripped someone off on the BenHeck forum at the same sort of time as he did on on my old forum - he was a trusted moderator on my old site and when I discovered he'd taken someone's money and not only didn't provide the goods, nor refund, had been lying about sending item out when he even later admitted he didn't, was lax on replies, and making lame excuses, I banned his ass and made it public that he couldn't be trusted - he went under the name of mrafterfx; turned out this wasn't the first time he's conned people unfortunately. He was keen on Battlefield 3 at the time and we tracked him on that forum too before he did a runner there as he knew we were on his heels. I can't remember if the name mentioned here was him or not though, was several months ago.
Problem with commissions is you have no way to trust someone to fulfil a commission unless their reputation is solid and they have something to lose - if they are just "a regular member" then they can leave the forum concerned and run, and not be tracable. By definition, if someone has something to lose (eg reputation) they will be trustworthy - or take your chance with "regular Joe" - we aren't talking about small change here after all.
Personally, I only take on commissions if i'm prepared to do them, only take deposits when i'm ready to do work (even if the buyer is keen on paying in advance - i've lost orders in the past due to this honest policy, but hey!), and if (like now) i'm not taking on commissions (apart from one in the next months, will do more later in the year) I tell people that. They see worklogs of progress and even offer repairs for systems i've made; when items get sent I forward them a copy of the postal details as proof of dispatch and conclusion of transaction. That's what I mean by reputable.