I vote for "I'd pass" for the following reason:
1. Sleaze factor. I'd be concerned that there are other things that the seller is "forgetting" to tell you about that you won't discover unit you start taking things apart. Things like cracked/chipped gears, etc. will ruin your day
2. This is a 30-year old Asian machine. One thing I learned with my old Jet 1024 is that the label doesn't tell you anything about what it actually is and where is was build. I had dozens of people contacting me about the 1024 (I posted about it on my forum some years ago), and it appears that there were at least 6 different manufacturers that made variants of this machine with incompatible parts. Even though Jet still has records of my S/N, they themselves had no clue what the machine was and the manuals (3) that they sent me were all different and din't apply.
3. Quality of Taiwanese machines from 80's and 90's was about as hit-and-miss as that of todays Chinese machines. Unless you really need 13x40, $5000 can get you a new Grizzly 12x32 gunsmith lathe that will be probably no worse than what this lathe was new, but that will get you warranty (with returns and exchanges), parts availability, etc.
If you are really set on this one, though, check the following things:
Find a way to check the gearbox (as in ask to look inside and inspect the gears)
Find a way to check that gears in the carriage are not messed up. With a lathe of this vintage the power feed worm gear can be worn out and the half-nut can be chewed up.
Check that the spindle taper is intact and is running true
Run the spindle for 20-30 minutes at high speed and check bearing temperature and listen for new noises
Check that the tailstock not high or low (i.e. the centers are at the same height) and the quill is not loose (especially when at the outer extreme of it's travel).
Tighten the gibs on the cross slide and see if it's not binding. The ways on these machines are hardened and take some effort to wear out; cross slide dovetails wear out fast, especially if the lathe wasn't kept clean and properly lubricated. Do the same for the compound as well.
Hope this makes sense
Yuriy