Thanks Lily.
Redirecting old sites is unlikely to give you a rankings boost, especially if they have changed hands and have expired / dropped. You really need to buy them before they expire and don't change two many details.
Think about seoa-moz.groupbuyseo.org to a-moz.groupbuyseo.org - all the link juice have been transfered, and so it should, its just a rebrand, same servers, so owners, etc. But if they brought so ramdon site and redirect do they deserve the link juice? No, and google doesn't think so either.