I'm an exchanger, so this is according to what I've seen, and would expect, and also what I would report as advertising.
as long as you aren't posting in threads outside of threads where people are selling, I don't think it would be considered and advertisement. It really isn't any different then saying "I'll buy it", in my opinion. I don't think anyone would report you for it, and most would probably be glad to have the offer. The only time I would report an add is if it was directing me to a shop etc.