Who is doing a favor depends on what is produced. For items with a strategy of long production runs, being able to sell surplus equipment (which might be due to substitution of upgraded or new equipment, or simply an active arms export industry) is to their benefit as well as the buyer. If such equipment is fairly generic at that time (magazine bolt action rifles, for example), it's a favor to the seller in that he can usefully dispose of surplus. If preparing for war and selling (or lend leasing) equipment puts you behind your build-up schedule on complex equipment with long tool-up and build times, then it may indeed be a huge favor.
In coming up with simple rules relating to this, it is probably best that any error come down on the side of high prices for recipients, modest volumes, and to be less restrictive on purchases by those countries without other good options due to tech, small of inefficient industrial base, or time pressure (threat, or tension). One may refer back to the case of Brazil where there was great pressure to acquire equipment.