if the tubes in there are say, a nice fresh set of New Old Stock Mullards, matched and wonderful....and will go into your next amp, or another amp you have, then yes, you buy a nice new set of Electro Harmonix and stick em in instead.
Most buyers don't believe you when you say "just put new tubes in" anyway, so why shell out big for nothing.
Ultimately, if you feel you want to, then do it with a tech and get him to invoice clearly, for proof.
But then again a new set, if it will make the amp sound better during demo,might decide/make the sale?