Now the thing with spam is not ok.

But buying sigs in relevant forums is not immoral. Forums could counterreact on this and put nofollow tag to the links in the sigs.

It would be ok, since it would not affect the algo, and at the same time, they would (or would not) bring in visitors to those sites.

To clarify myself the only accepted way of this is:

- by buying sigs in relevant topical forums. For example in a travel related forum for a travel site. It does not annoy users.
- by buying them for the visitors that would come through those links and not for the algo (this could be prevented though by forum owners I guess)
- by not buying newly created accounts, and by not supporting the spammy posts these new accounts make.

Spamming forums and then selling the links is not ok, but buying a sig link from a reputable member of a forum, a nofollow sig link maybe, would be acceptable, as people look for what those members have in the sig, and thus they may become prospective visitors of the site that is advertised.