I wouldn't buy a used helmet. A brand new HJC at half the price will protect your head more than that lid if it has been dropped, and there is no way to tell (besides getting it x-rayed, but when that is done, you are probably at the cost of a new lid). Getting your head smashed in is not something a doctor can usually fix, one chance only. Is your head only worth whatever the price difference is between used and new?
If the helmet looks sound, and the nicks look minor, I'd say 150-200 bucks. New Quantums can be had for about 300 on ebay. Personally I'd only want the helmet if it was one with the removable liner which was added last year.
i landed on my head after somersaulting over the bike doing a stoppie. only a few nicks on the helmet, but my neck hurt for days. usually helmet blows are quick - don't see many guys sliding on their head. just a few nicks can mean a hard hit, and just one is required to make the helmet useless.
if price is ever an issue go for a helmet with fewer features. the kbc racer-1 has outstanding reviews and can be had for $250. 100% guaranteed to protect your head. once a helmet has hit, you may as well throw out those dot and snell stickers. it probably wouldn't pass.
Don't buy it. The the liner that actually protects your head from being jacked up in a crash forms to your head, and if it's formed to someone else's already its not going to provide your near as much protection as you need.
That is also why you shouldn't let anyone else wear your helmet.
Besides, do you really want to smell someone else's helmet funk?
Get a cheap-o snell approved helmet if you can't afford a new one like this.