Ummmm.. buying a cooler vs having one pre-converted... that difference could be worth an extra $30. SS ball valves alone are $25 or so, and it's already been done for you. A corona grain mill isn't optimal, but if you don't have one, that's another thirty bucks. Ale pales are sorta junky, but new ones cost twenty bucks. So if you have a use for them, that's another forty (new price). And there's no way you are going to acquire a keg and convert it with a SS valve for $35. $75 is more likely and $100 isn't unreasonable if it is decent. If you have a free keg, you'll probably spend $25 in materials just to crack it open (grinding wheel, lumber to build a support system, etc), and if you need to buy an angle grinder, there's another $50 expenditure, just to get a crappy one you'll hate using, and you'll spend 6-12 hours doing it and making it pretty (for a first timer). So $500 doesn't seem unreasonable if you want to take the seller's path to AG and if your time has any value. There's better ways and better equipment for sure, but $500 would still be a good deal if you want what the seller is selling.
I'd look at it this way:
Malt Mill: $125 (better)
DIY 10gal MLT: $95 ($40 for cooler, $20 for false bottom or whatever, $35 for ball valve assy)
Pre-made 15gal Brew Kettle w/ ball valve: $120 (DIY is cost of keg, plus $35 ball valve assy, plus $25 materials to convert the keg, plus whatever your time is worth)
9gal SS pot: $80
3 kegs: $120
plate chiller: $80
plus capper, pales, carboys, etc
I once spent $10 on a SS bathroom hose, plus $4 on a brass barbed fitting, plus $10 on a brass ball valve, plus $10 on misc hardware and spent 6 hours acquiring the parts for, and making, a system that I tossed aside for a $20 bazooka tube and $30 Cooler conversion kit with SS ball valve that took less than 30 minutes to put together. How handy are you? What tools do you have available? How much time are you going to spend to make a single extra run to Home Depot? Are you going to make one trip to the store and put things together right on the first try? Are you going to pay shipping costs for a single stupid component that you can't get locally? What is your time worth? I consider myself a pretty handy guy, but I don't have the equivalent of a machine shop at my disposal. I bust out each tool from a tool chest, and using an electric tool requires cleaning some space and running an extension cord.
As an experienced brewer who recently reassembled and reacquired quite a bit of hardware after a brewing hiatus, that'd be a tempting combo deal to me (if I wanted the kettles), even at the asking price...
When being involved in a hobby with a DIY mindset, it's easy to be penny-wise and pound-foolish. It's not a bad deal if the equipment is in good shape and seems to work well and suits your needs. All the little stuff is done for you. That $1.50 gasket that you forgot to buy, that requires five dollars in gas to go get, and 2 hours to measure and fit up and test is already assembled and working there. I'm all for DIY, honestly, but don't kid yourself about the costs in trying to reinvent the wheel.