Let's hope you have got a good lens and maybe lenses are much cheaper in the U.S. but I wouldn't sell my El Nikkor 2.8 for $20 unless I was in debt to the Mafia and the Goodfellas were surrounding me and had taken a liking to it . These lenses never sell for anything like that price in the U.K. More like $60 and even then a bargain

Might be worth getting an expert to look at it