Is it a Bush Hog brand, or are you using "bush hog" as a generic term? $3500, even for a used one is cheap. I'd make certain it's in decent shape before buying. They can get to be money pits if there's problems. In addition to checking driveline slack for wear, look at deck hinge condition. Pay close attention to main drive shaft,especially flex points nearest the tractor end. Main (center) gearbox is less of a problem than outboard gearbox(s). Check oil/grease in the gear cases. seals can fail. Sometimes people fill them with heavy grease. That MAY indicate seal failure that may have caused significant damage to gears/bearings BEFORE filling with grease. Always a good indicator on comercial equipment, look at the hitch pin hole. Is it worn excessively? An elongated hole will indicate LOTS of hours of use generally.
A NEW Bush Hog 2615 Legend mower is north of $12,000 (with a GOOD price) so a commercial mowing contractor who's bought several, selling one for $3500 would probably indicate it's been worn hard. (2715 Legend even higher)