In theory, someone that is really determined to rip you off can generally get away with it. You can threaten to call the cops, or to report the gun as stolen; but srsly, you're hosed.
However, the reality is that bad deals in the EE are vanishingly rare, and the opposite is generally the rule. Look at the person's trader rating, drop a PM to a couple of the people s/he has done deals with and then just do it. Sure someone with a bigger trader rating is safer, so someone like Doug, with a trader rating of 400+, ... just send him money for the heck of it, he'll make sure you're happy. Someone like me with a trader rating of less than 20, you'll probably want to contact a few of my 'customers' before you send that cheque.
And someone with a zero trader rating?, first make sure it's not zero because of bad trades, then if it really is a noob, you're just going to have to trust your instincts.
Here's a hint tho', actually get their home phone number and talk with them first. That will usually clear-up any potential misunderstandings (which are the vast majority of 'bad trades' IMHO).