There are plenty already, by all sorts of players. However the issue is search - which is probably one major aspect of the "friction" mentioned in the parent post:
1. Patents are often not exactly written to easily give their secrets (I would say there are specific forms of patent legalese).
2. You need (costly) experts to look at potential offers.
In addition, there are more disincentives to "easy" IP transactions:
1. Even when you've found a matching patent it doesn't mean that you can just pop out a product. Most of the actual work is still ahead of you.
2. Pricing and contract negotiation is rather difficult and costs as well.
3. The existing marketplaces are mostly rather small with the largest having maybe 20-30k offers. Just not enough to impact the myriads of problems available...
