Where can I buy uninhabited islands in the Seychelles or Caribbean?

All naturally formed landforms (e.g. reefs, cays, banks) in those areas is already the territory of a country, with no terra nullius remaining, although there are a couple disputed claims. You want to search for the phrase "buy [country name] islands".

Crazy what people will do for IPv4 space these days

If you had a carrier strike group in the area, you could try (after the example of https://en.wikipedia.org/wiki/William_Walker_(filibuster)) getting your islands for the iron price?