RIPE is the RIR with the most liberal transfer policies. In RIPE, legacy address blocks can be bought and sold (even partial blocks) without a needs-test and without the requirement that the recipient become a RIPE member or that the addresses lose their legacy status.
RIPE provides some services for market participants, including a list of recognized brokers and a list where buyers and sellers can post need and availability. RIPE also provides a log of all intraRIPE transfers and interregional RIPE transfers. IPTrading is a long-time recognized RIPE Broker.
IPTrading.com is also a RIPE LIR member and has access to the RIPE Transfer listing service, which is restricted to RIPE members only.