How screwed am I?

Maybe I'm confused here. You have a title and such. But you aren't listed as the buyer? Who's listed as the buyer? Do you have anything proving what you traded to him?

You guys got some crazy rules down there. Up here you don't even have to have the registration. As long as the person listed as the seller is the last registered owner in the registration it's good. You can write a bill of sale on a back of a napkin if you want. Seller. Buyer. Witness. Date. That's all you need. Obviously more is better. And every registry hands out bill of sales for free. And every registration comes with a bill of sale on the back. The lack of paperwork has made for some interesting cases (guys buying cars then getting drunk and wrecking them and only claiming they were loaned) and such. But it's a lot better than having a 100 forms to fill out. It's buyer and seller beware.

As for what to do. If you just need a bill of sale from them to you. Like RRR said, make one up. They did sell it to you. You did give them something for what you got. You're not cheating anyone. You're getting what's yours. Blind squirrel might freak out over that. But fact is you completed the transaction in good faith. Plain and simple.