I have a chance to buy a Atlas model 618 lathe in good condition for $500.00 is this a good price.

How much tooling comes with it?  Chucks, tool  posts, bits,  many times it's the add ons that are the expensive part on running a lathe.

I'm not totally familiar with that particular model but it appears to be one that requires change-gears for different threads.  If any of those gears are missing, that makes a HUGE difference in value.  As TP mentioned, extra tooling coming with it can also significantly increase value over a bare-bones lathe.  

What should normally come with it in addition to the change gears is a 3-jaw chuck, a 4-jaw chuck and a face plate.  You will eventually want all 3.  The various tailstock tools like a drill chuck and live or dead centers are good to have but not that costly to replace.

That $ 500 you mentioned is not that far off "normal" for a complete working lathe of this calibre---Only a bargain with some good extra tooling and overpriced if incomplete, worn out, or if there is damage to any of the castings. 

If the seller is tossing in things like drill bits for "extra tooling", check the country of origin (usually imprinted on the shanks).  Chinese stuff has ZERO value there so don't let it sway the price upward if that's what he has.

And....be sure and consider that you'll put several hundred dollars more into your own tooling for this lathe.  You can probably spread that out over time but it should be at least on your mind when purchasing.

A steady rest , and follow rest are also nice to have.

How much backlash in the cross slide and compound at each end and in the middle of the travels?  How good are the ways? I have seen them worn so much in the middle that it was swaybacked. Can you do an indicator test with a test bar?