How much is a brand new one? My rule of buying things used is that I offer 1/4 of new value or lower. Say it's a $75 new.. You should be able to get it for $20 as long as it looks like it is decent shape. If not then lower the price. Sounds like he is not sure if it even works so offer him $15 if you can. Gotta think of it in 2 ways... 1 he's not going to be useing so he needs the money and space..... 2 You need some type of filter so go with what is out there to help you get a better one in the future, or at least last you a short time.