I think you should report them,or you could give them weapons potions or armor for your cash back
Extortion? DON'T DO THIS...
Only two things you can do in this regard...
1. PREVENTATIVE: Don't open yourself up to these types of scams by putting things into a trade box and pressing "check". Only trade with people you really trust (and remember there is never a guarantee that your buddy is truly 'trustworthy,' so err on the side of caution). AND, inspect all your trades CAREFULLY before accepting. There is currently no 'safe' way to transfer your items between two of your own characters, with the exception of having two accounts and two devices (to do it yourself, manually). Unfortunately, an in-game item-transfer between characters feature WON'T be in this update, although single character 'banks' will. With this in mind, if you MUST hand over an item to a friend so he can pass it to one of your alts, USE EXTREME CAUTION, only do it with those you REALLY TRUST, and be prepared for the worst if things go sour... (an accepted trade is essentually a contract between players) You could also try asking for collateral from the other player, but good luck finding people willing (or able) to do this!
2. IF IT HAPPENS. Report it. Send a private message to Support indicating the circumstances that transpired (as you remember them), give the approximate date and time (with your timezone) of the incident, give the names of the party/ies involved, and wait for them to investigate/reply/rectify the situation. Again, DON"T GET YOUR HOPES UP... Depending on the situation, the admin may not be able to do anything to help. REMEMBER that by clicking that little check box, you are essentially 'signing' a contract with another player. (see point 1, above) If they are able to help you, they will. You do NOT need to send them screenshots (unless they ask for them) as they can pretty much garner all the info they need from the logs.
Hope this helps.
Shaggadelic