i'll take a general stab at it. i did a deal a week/two weeks ago, more or less trading a-ram for beltran, after beltran had his 3 steals in 2 days. in our league, a-ram was a high 4th rounder who was (at least) matching expectations and beltran was obviously a mid-first rounder who was not. and that had about the right feel to it.
as general advice, i would think buy-low players would be fair exchanges for:
* players of similar expectations who experienced similar woes (beltran for helton two weeks ago would probably be considered fair and interesting to both sides)
* players of slightly lower expectations (say 2-4 draft rounds) who are performing solidly to slightly above
* players who came from nowhere who are just dropping everyone's jaw
if someone is trading away a pre-season stud, they're probably still holding onto some hope that they're going to return to form. so to compensate or make it interesting for them, if it's going to be lower value, it has to at least be a solid acquisiton. and if it's going to be a much lower value, it has to be a spectacular one because there's always the fear that you're buying high and all of a sudden brian roberts is going to turn into, well... last year's brian roberts.
that would be my broad take on it. it also helps if you can address a specific need they have or if you can tell that they're just getting fed up watching this bum do another 0-for-4 night and are getting desperate. but i would say that would be a guideline that you could at least get people interested and to the table with.
best of luck.