I'd have no problem buying used equipment (cars, trucks, tractor or implements). In fact, my 670 was used. I've got two used implements now:
A) Befco finshing mower (purchased when I got the tractor) which has been real nice. I've got to replace the belt (Thursday's task), but it does a fine job cutting.
B) 6' Back blade which is ancient (40+ years, I'll wager). Other then an overall look over and make sure everything moves or rotates, not much can go wrong with a back blade.
I've been looking for a cutter (only saw a few 48" units so far)...this is what I've been doing:
For the mower or rotary cutter, gear box is lubed and turns freely and the blade(s) rotate when the gearbox turns. If possible, try to shake the blades to check for worn bearings...look at the blades while you're at it. If the owner doesn't mind, pull the belt cover(s) off and examine the belt for tension and wear. Wear wouldn't be a show stopper, but since a belt will cost $30-$50 ($38 for the Befco) you might get a little more off the price. Visually examine the grease fittings. If dry, the unit may not have been maintained well. Also, check out the caster wheels for free movement.
Obviously, this assumes the mower/cutter is not hooked up to the tractor!! I'm also assuming the PTO shaft is attached to the implement. Speaking of the PTO shaft, make sure the release mechanisms aren't rusted and are functional. Slowly rotate the PTO shaft until you feel the resistance of the gearbox, then reverse rotate....that should give you some idea of the wear in the U-joints.
If the mower/cutter is rigged to a tractor, ask if you can see (and hear) the implement in action.
I'll tell ya one thing that always impresses me...if the seller offers up the Operators Manual when you're looking at the unit.