The main ways it goes wrong are paying for a block the seller does not actually control, skipping the reputation check and inheriting a blocklisted range, and wiring money with no escrow. All three are avoidable with a process. Start by defining exactly what you need in terms of size and region, since registry rules differ, then verify the holder in the registry database before any money moves. You can
Buy IPv4 addresses safely , through IPv4.online, which is the route we took, and having the paperwork checked by someone who does it daily was worth it. Budget one to two weeks for the RIPE transfer after the documents are clean.