plus,if someone is out there to rip somebody off,banning them off a forum isn't going to stop them,theres lot of ways to gain membership back and hide your IP address if you so wish.
We have a monitoring system in place and without giving too much away - using more then one login on the same computer or from the IP address flags up a note to admin straight away. I can ban individual IP addresses or groups of IP addresses should I have reason to believe that someone is using more than one login, or is a previously banned user chancing his arm again. Yeah, if you are cute enough there are still ways around it (there always are) but most peeps still slip up at some point and then I spot 'em ......
I also have some clever software that allows me to find people with minimal information (just a phone number for example) so I do make an effort to find all and anyone that rips peeps off on my site. Some can still slip through and are not traceable - but it is only a few. We are rarely short of volunteers to call round to said address and collect the missing item, or cash if needs be once I have located them.....but that still all falls into the category of shutting the stable door after the horse has bolted.
The only certainties in life are death and taxes - with everything else care and common sense needs to be exercised, especially when distance buying from someone you don't know. Don't assume that just cos member x once rode with member y - they are both solid guys. Chances are they will be - but still it's better to actually BE sure than just think you are.
One of the simplest methods we found of reducing fraud, is to exchange addresses before even using paypal to send money, someone giving you their home addy is a lot less likely to rip you off than someone that thinks they are invisible...if they still do rip you off after exchanging (and it's dead easy to verify names and addy's are legit), well then you can usually fall back on the volunteer call round method ....
