I never have removed the black Sikaflex, but the leak hasn't returned, and no one can see it, so all is well.
Palm', something may leak on the Safari you are interested in, and if it doesn't leak now, it will sometime, somewhere. Get out the garden hose and spray it down and look for leaks.
Check around the awning arm supports (4 of them) to see if they have been re-sealed. If they haven't, you may have a leak under the vinyl and/or carpet because they leak easily. Look for water tracks in the dust on the inner walls (fairly easy to see if you have the aluminum interior) including in the cabinets and behind drawers. Look for cracks in the skylight and around the fan housing. Walk around everywhere to check on soft spots in the floor.
Maybe someone near you will inspect the trailer so you won't be overwhelmed. This is the nicest guy in the world to bring the trailer to you, or he has reason to really, really want to get rid of it. Check it over thoroughly. Make sure he has a valid title and registration. If he said where he lives, Google him and the address and confirm it. Sorry to be so distrustful, but Airstreams do get stolen. Check his ID against the documents and run a check on the plate number when he arrives.