Keep it or get rid of it

First of all, what smells like gas to one person doesn't always smell like gas to another, nor does it even mean it's gas to the person it smells like gas to. Many people pull into a shop saying they smell gas that actually turns out being a coolant leak, A/C leak or even mildew built up in the system.

So I can't really guess at to what is causing the smell you are smelling. But I do know anytime there's ANY possible chance of gas or exhaust fumes leaking into your car, you should STOP driving it immediately. Not only can this cause a fire/explosion, but these fumes can kill you without you even knowing you're dieing.

My advice to you would be to have the car inspected. Then make the determination on if you want to fix or sell.

It's not always easy locating the source of a leak. But because it only smells like that when the tank is filled to a certain point, then that narrows it down a lot. The leak must be somewhere towards the top of the tank.

Also, you could fill the tank up and then raise the vehicle and look for where the leak is coming from. But it will still mean taking the tank off to pinpoint the leak or fix it.

The decision to either fix or sell can only be made by you.