Too much depends. If the bike doesn't fit your intentions for it or if it needs new parts such as wheels then $75 could be too much. If it's truly nearly new or well cared for (more likely for an 18 speed than nearly new, I think) and you need something to tool about on an entry-level bike could be perfect.

I'm not overly familiar with Diamondbacks and the ad wasn't very specific. The blurry picture of the bike itself appears to be either an older style or one of their entry level versions. A little more research might be in order.