I have a 93 Civic Dx, and I am selling it. So I got it inspected, and the minute I started driving it after picking it up from Pep Boys, the speedometer went from normal up to 80, then back down to zero, then up again, then normal, etc. Never happened before, only the day before I was to sell it to a buyer with cash in hand! From what I understand, this is fairly common, but can be caused by a number of different things. My mechanic says I need to replace the vehicle speed sensor. But that's $200 plus, and no guarantee that it will fix the problem. The car is only worth $1000. Is a vehicle speed sensor (on the transmission) difficult to install? What else might be the problem?

Thanks.