I would run far away from this.
I've sold one car on ebay, and basically had the other party give me a $500 deposit via pp, and the balance when he came to get the car, which actually was via a certified check. However, I don't suspect a scam when the guy and I talked through email and phone calls and then he literally drove from out of state, with his wife, then spent another 2 hours looking the car over before finalizing the deal. It was only for $5k too, so it seems like a lot of hassle to try to scam for that small amount. In the end everything went through fine.
Now had he said he would show and sent someone else, i would have cancelled the whole thing.
In your case i'm surprised the guy didn't want to also paypal you $25,000 and ask you to give the car and $5000 cash back to the "agent" lol.
Now that I think about it, I wonder if those "online background check" things may be worthy in a case like this? I think you can spend like $50 or something for a month of the service and then if any shady type of thing comes up, you just ask for all their info so you can verify it. If they are legit, it shouldn't be an issue.