There are currently no ways to 'mark' a trade as problematic. There is, however, a board where you can discuss such trades, which IMHO may be a better way to handle them. I'll move that topic over there once I finish my answer.
The Dirt 3 problem is pretty well known. I'd want to say, I know of *no one* on other trading sites that would willingly trade it without being aware of its 'market value' (IIRC, it's because of a problem with real-life wheels not working very well on the Complete Edition).
So, if someone traded you Dirt 3 Complete for something of low value, it's likely they'd be unaware that Dirt 3 is 'different'.
Here's what I'll do: I've tagged your wishlisted copy as 'Do not offer Complete Edition'. This notice will appear on game pages, as well as on your wishlist. I need to enable two-way tag lookups for that notice to appear in offer matches and offer creation pages. But that's a start.
I'm gonna assume the other trader is honest and you are too, and no one should be punished for the confusion. There aren't many people who have Dirt 3 as available, and from what I remember, at least half of those are people who actually own Dirt 3 Complete and think they're doing wishlisters a favor by adding Dirt 3 to their tradables because they don't know it's wanted for different reasons. (Yes, I did that honest mistake too.)
Anyway-- Dirt 3 Complete is so well spread across the trade market, it has little to no value at all, and thus the other trader shouldn't bother.
