Not a good idea. In order to sell on IM you need to have someone who can answer any questions. If the potential buyer has a question, the bets a bot can do for him is send him to some strange link to a page on which he might not know what to look for. Selling on IM or IRC is as difficult as a cold call. I did this when we first started out, and decided that it wasn't worth it.
Quite frankly I block everyone I don't know from contacting me through IM. If you're not on the list I don't see your IM. Not to mention I would be particularly annoyed if a bot tried to contact me and make a lead out of me. I view this as an underhanded marketing tactic and would never do it.