unconfirmed reports. Gonna need to wait to see if he becomes marked

Even if they are not marked, if the history obviously shows that the unusual is from a scammer you might get banned. Outpost banned those people cause they traded with scammers. That means you should be cautious even if they are not marked. Just check the unusual carefully. If the unusual is clean, you can trade with them(assuming they are not marked on steamrep which is the only reason that you can think of trading with them).

If it is a scammer, marked or not, main account or not, you may not trade with them. If the items are hijacked/scammed and you know about it, the same applies. If they have multiple pending reports you should check the reports and, if valid, decline the trade.