I'm sorry, did I do something wrong? I've never done this before so if i have done something I shouldn't I apologise.
No it's alright. You're free to trade with someone through DMs if you see them make an offer you're interested in (in the case that the OP doesn't accept/reply to said offer), but it's generally frowned upon to carry out the trade in the thread that the OP made; it's
their thread after all, and all trades on the trade should only be directed towards them.