A cheap block with blocklist entries usually isn’t a bargain. I check the range on several public blocklists, look at its history in routing data, and ask the seller what it was used for before. On the InterLIR IPv4 marketplace the pitch is ‘no abuses’ and networks from real owners, which is the kind of promise you should ask them to back with details. I’d make delisting a condition of purchase, or walk away. Test the range with a small announcement before you commit. We made the second purchase conditional on a clean result after thirty days, and the seller agreed, which says a lot about who is confident in their stock. Also keep screenshots of the blocklist checks from the day you looked.