Whether it's a verbal or written agreement between two parties, bartering involves a good deal of communication such as arranging a meeting, e-mailing, speaking on the phone, and it certainly doesn't hurt to ask for references.
Remember that the whoever you deal with has the same concerns you do. If you're suspicious or doubtful, then it's most likely for a good reason. If you don't feel comfortable with a certain situation or if you feel that the other party is doing something illegitimately, it's perfectly acceptable to cancel the deal.
I would suggest however, that you inform the person that you've been negotiating with, that you're no longer interested. This saves them from waiting around for your response.
If you've already made an agreement with someone, it is not okay to change your mind just because someone else has offered you something better. A deal is a deal and you should honor it, consider it an electronic handshake.
If the other party has already followed up with his or her end of the bargain then you should return your offer in a timely manner. Procrastinating will only leave the other party questioning their trust. If something urgent has come up and you can't honor your end of the deal right away then you should inform them that you've been busy and that you have not forgotten them. It's not necessary to go into great personal detail!