Marc it would seem that in the case of sellers or buyers having fraud charges brought to your attention the reasonable thing to do would be to "temporarily" suspend their trading privilages ( as e-bay does) until charges can be reasearched. If they are unfounded then re-instate them. It is always better to protect the guy getting ripped off than the guy doing the ripping.
Also I see RCU wanting to absolve itself (from what I have read) for any responsibility I.E. caveat emptor and that would be more undestandable under the old forum based model but now you have turned this into a pay for play system which in my mind means you DO have a responsibility to police these issues in a timely manner just as Ebay does. I know you aren't getting rich on this stuff but it is your baby and you have to take care of it. It is no longer like a newspaper classified section as you have control over every step of the process now. Just my opinion.
I was not a fan of the "credit card" registration (although I did provide it)to buy since I use paypal as a saftey net of sorts but I feel sellers should be required to provide some sort of verification since the amount of some of the items sold here is very large and sellers should have a source to reimburse anyone who does not recieve the goods they have bought and payed for.