Its pretty common for
SEO companies to use forums as part of the link building strategy for their clients anyway - they're just not so transparent about it as to sell forum links in blocks. Its a method of holding an ace over the customer as well - since once the customer stops the contract, some
SEO company's then just take down the links, with one easy edit. Personally I'm not a fan of this and any links acquired over the term of a contract ought to stay up in my opinion
Selling forum links like this though is not the best way to go about it, since the links aren't really anybody's to sell. Whereas paying for an
SEO service to find relevant links - including on forums might be seen as a little more legit. Its all about packaging the service.