ok now is the point when the time value for money thing applies
if you are in dire need , need to buy a car as soon as possible , and have not came across a similar ride yet , and fear that such exceptional condition would be a hard find..then throw an offer of 17,80... no big deal.. if he doesn't lowers , then leave it...
now if you can wait , search the market more, so u will definitely get one below 18 that's for sure , with a strong probability of finding one around 17.5 range.