IANAL but I think you can use ordinary civil law to enforce the contract made when you have a situation of "You offer - I accept" as long as you have evidence of:
1. Offer is made by a specified individual of a specified item at a given price.
2. Offer is accepted by a specified person and payment is made.
Think about what is in the typical bitcoin uri.
Say I want to buy a copy of Cryptonomicon by Neal Stephenson from Bitcoin Books and they give me a uri offer of:bitcoin:15ZLe7GCAfdTTMMkbm38KTtahq9y549rB2?amount=3.25
&label=Cryptonomicon, by Neal Stephenson
The only thing missing (for later court action) is the identity of the person making the offer.
You could do this in a couple of ways:
o Bitcoin Books offers a https service that validates the bitcoin URI is one of theirs (also prevents man-in-the-middle attacks)
o They sign the URI with a "trusted" private key and you check it against a public key.
If they failed to deliver (in the UK) you go to Small Claims court and say:
"Bitcoin Books offered my a copy of Cryptonomicon for 3.25 BTC. Here's the proof they made the offer. I paid. Here's the blockchain proof. They never delivered. I want my money back".
No different to a million other non-bitcoin claims.