Firstly, I'd definitely suggest not dealing with other members directly.

A signature campaign is a decent idea, you would be best of if you messaged escrow.ms (https://bitcointalk.org/index.php?topic=235452.0) who would be able to assist with escrowing for a signature campaign, or EvilPanda(https://bitcointalk.org/index.php?action=pm;sa=send;u=134217) who has hosted quite a few signature campaigns in the past, and currently is hosting GAW Miners, a pretty popular signature campaign.

Alternatively, you can try your luck at the forum adslot auction(https://bitcointalk.org/index.php?topic=838764.0) which will get you official forum slots. Lot less hectic, allows banners instead of just BBcode too.