Realistically man, u gotta expect to get sum duds when u play in the "used gear" market.
Having said that, ive bought
8 vintage lens in the last year (all under $100) and only
one has been a dud (& the seller took the return, so no harm done)
Its just a bit of a gamble (which is probably why i enjoy it lol)
but it really can pay off!!
Conbined total cost of ALL my vintage lens doesnt even come close to the price of an L lens (tho neither does the sharpness of course).
Try googling sum vintage lens sites for specific info.
Dont be scared, just dive in, its only money....... lol

Ps. Caleb Pike at DSLRvideoshooter.com has an extensive video guide on nikkor lens, check it out!