In reply to Trevers:
Things didn't go so well here. We contacted the seller, but soon after the bike was listed as sold so has presumably been moved on.
Our first thought was that if we contacted facebook and explained the situation, they might be able to provide metadata on the messages linked to the post to enable us to contact whoever purchased it. However, there isn't a customer support service that you can phone or email. There is a "community support forum", which is completely useless.
There is the option to report the post to facebook for breach of standards. However, stolen items are not a part of their list of options. So I had to report the post for selling "drugs, guns or regulated products" instead, which was the closest match to "stolen item" I could find.
Facebook have replied thanking me for my "feedback" but predictably they say the post doesn't go against their standard, but maybe I can block it if I find it offensive. I've responded with feedback explaining the actual situation, but I don't have much faith it'll be read by a human being.
In summary, facebook have no customer support and seem to be perfectly happy that their product is being used as a platform for the sale of stolen goods.
Our only recourse, it seems, is to start directly contacting the seller or his girlfriend in the hopes that he'll be sufficiently scared that he'll divulge the information on who purchased the bike off him.