Thats about 7 months ROI (using 36/day) which seems about right at the current going rate. I would test each rig before hand to ensure everything works.  Depending on the internals you may be able to build your rig cheaper. Depends on what PSU, CPU, RAM that he is using. You may be better off piecing yours together and then building it yourself. Make sure you take into account the electric cost you are paying and update that in whattomine as well as ensuring you have enough power at home to plug all these in.

RX 480 cards have been mostly out of stock for a long time now, so there's a chance that the rig was used for more than 2 months (that's of course if you don't know the guy personally and not sure whether he's telling the truth). You might also want to find out whether it's 1060/3 or 1060/6 cards (3 gb or 6 gb) and 480/4 or 480/8. If it's 1060/6 and 480/8 then it's not a bad deal, but if it's 1060/3 and 480/4.. well, it's still alright, but not that good.