I've got a pair of Uberti Cattlemen in 44-40 that I shoot in CAS from time to time. They're good solidly built guns and should last you a long time. Even with a 44 Mag cylinder, I think $500 is a little high for a used one. New in the box single cylinder models go for $500 all the time. I'd give it a good inspection to make sure it had no problems, signs of excessive pressre or wear and do both cylinders fit? Then, if everything checked out I'd offer $350 and go as much as $400.
But, I'm a cheap..........er frugal
Fingers (Show Me MO smoke) McGee - AKA Man of Many Colts - Alter ego of Diabolical Ken; SASS Regulator 28564-L-TG; Rangemaster and stage writer extraordinaire; Frontiersman, Pistoleer, NRA Endowment Life, NMLRA, SAF, CCRKBA, STORM 327, SV115; Charter member, Central Ozarks Western Shooters
Cynic: A blackguard whose faulty vision see things as they are, not as they should be. Ambrose Bierce