Being a Nikon user for over 35 years I have a great deal of knowledge on their systems.  On eBay there are hundreds of good deals and just as many questionable deals.  I know I have purchased over $3000.00 in used gear over the years. A manual camera means just that, manual.  Manual, film loading, focusing, film advance, lens setting and shutter. 

My simple rules of thumbs are as follows.

No photos of the item, no sale.  The more pictures the better. Be careful if pictures are dark, out of focus or blurry.  The seller many be hideing something.

Feedbacks too many negatives stay away. Watch out for in the descripion, "..needs a good cleaning", "...needs a battery but works well",  "..a work horse", "...pro user", "..it belonged to _________ and we are selling it for him/her",  "...plenty of life let in this old camera".  These should be RED FLAGS.  Ask the seller questions, lots of questions!  Ask about a return policy.  

The old Nikons F, F2, F3 etc. were built like very well and has heavy as a tank compared with today's digitals.  

Have fun with you Nikon 35mm SLR. I hope this short guide helps you in your chose of Nikon 35mm cameras!