I don't believe you can access sold items like you could on the old classifieds. Typically, I look at the seller's post count/reputation in the main sections of the forum (though by now I recognize most of the regulars).

There is a feedback system in the classifieds, but it doesn't seem to get much use.

I agree that it is a good idea to contact the sellers if you are unsure; that's always a good first step.

I would also like to add that I have never had a negative experience on SRP either, be it buying or selling, and I've been rather active in both the old B/S/T and the new classifieds.