Any and all machine tools need to be inspected. Check the ways for wear, and rust. Check that the gears are all there, or it shifts if it's an enclosed gearbox.
Basically make sure it's not worn out scrap metal and everything
I’d get it if it was anywhere near me.. but I’m more likely to throw cation to the wind when it comes to things like that on my “one day” list.
heck I got a little unimat 1000 desktop lathe I spent 450 on.
For $500 i would say it's worth a look, but sounds like there's no point if you don't have space for it. There are work-arounds to make up for the long head stock for general use. But it's still not an ideal lathe for "precision" gunsmithing.
I would be really careful in inspecting the gearing in the thing, both drive and all threading gears, cross slide, carriage, everything.
"ran when parked" is always iffy, especially if it's missing parts.
If it's still in production and you can for sure get parts it MIGHT be worth looking at. Just know, like a car, it's less expensive to buy it whole and complete than to assemble the parts yourself.
An 18x40 is not going to be easy to move.
If it runs it is definitely worth $500 even if it never gets the right tailstock.
I have not modified tailstocks but I have made a steady rest for a 12x36 Clausing.
If you are old, worn out, and rich, forget it. If you are young, energetic, and low budget, it is great opportunity.
If the guy couldn't be bothered to clean it off before he took pictures, chances are he didn't bother to take care of it either. Unless you really want another money pit and mechanical headache, hard pass.
I sold my business and have Clausing in Washington State that I am going to sell that I purchased about 7 years ago and never ended up needing. Got 13k in it but am going to sell it for 7500. Has 3,4,5 jaw chucks, face plate, DRO,swiss tooling and aloris tool holder under power in my shop. PM me if you are interested.