As I mentioned earlier I have a 2001 Ford Focus and for the past month I’ve been smelling gasoline in the interior of the car, espically when the heater is running and the car has just started. My mechanic, as yet, had been unable to discover a source of the gasoline fumes. The fumes are so strong that if a source cannot be found the car is, for all-intents-and-purposes undrivable. If this problem cannot be ressolved, there is a real possibility that I’ll have to get a new car. What, if any thing, do I tell prospective buyers, should I decide to sell the car?
First, I would trade mechanics before selling the car. Second, if it’s as bad as you say, you won’t have to tell them anything, they’ll figure it out on their own. But if you must explain it, just tell them it has a fuel odor you haven’t been able to track down- it’s probably an easy fix you just don’t want to be bothered 
You might might need to find a different mechanic. One that has a ‘sniffer’ for hydrocarbons. These wee used to emissions test pre-1996 cars.
Theres a little rubber hose on the back of the engine on a 2000 Focus that can let crankcase fumes in the cabin,if it cracks(happened to me)
little more detail? rubber hose on back of motor? does it plug into the valve cover? pcv system? fords do have some emissions solenoids on firewall. perhaps a valve that routes gas fumes from gas tank to charcoal canister?