It depends on what was done during this "refurbishment" process. If some unauthorized person or shop did any repairs on the Mac, then Apple will most likely refuse further service on it and may refuse to accept it as a trade-in or give you a much lower value for it. Otherwise, it's likely that you won't have an issue getting service or trading it in. Caveat, though: we are all just fellow users here do not speak for Apple, so our opinions and experiences may or may not match what Apple actually says and does.
As to purchasing a used device in general, it's always a bit dodgy, but Backmarket at least seems to have decent buyer protection. But before you buy, confirm with them exactly what "refurbishment" means and what they will allow should the Mac you buy not be fully functional or as advertised.
Regards.