those are some low quality link... but as host298 said, it doesn't hurt i saw someone selling signature...i wouldn't pay a buck for it unless he is a very active poster in different high traffic forum. My personal experience. I used my sig to show my site on slickdeals.net. and i 've got around 200-300 vistor from that sig a day.
I think signature link is generate traffics a little, might be much more if posted in relevant themed forums. I'm not sure about value to search engine with signature links but in my opinion it's worthless. I also not sure that search engine can follow links with ? I think they can follow only url with <a href>. Any suggestion?? :D Cheers, Sxperm
So if ever you will be using this practice.Make sure that you have a very good reputation to that forum so that it will attract more user to hit your sig.Cheers!
or you could just register in so high traffic forum, then start threat attacking poster in forum..there will be traffic..