I bought Nikkor 24/2.8 AI and 35/2.8 AI in the last year. Obviously I only have my experience as a buyer to offer, but for what it's worth:
The 24/2.8 came from a private seller in Venezuala, bought through an ebay auction. Great condition, no wear or marks on the body, no fungus or scratches visible in the glass. Looks like it's just come out of the box (but no box or manual supplied). Cost £60 plus shipping. I had to wait for a while to get one at this price in this condition. Maybe the seller's location put off other buyers.
The 35/2.8 came from KEH. BGN graded. Glass has no visible problems, but body a bit scuffed and a few minor chips in the finish. Cost $49 plus shipping.
HTH
Ian