I think in a situation where a player is canceling an account, it would be reasonable for them to have the option of donating their perks to another player, or even distributed to their squad mates. I really don't have advocacy for some system to buy/sell perks
The buying and selling would be unseen, in the form of private messages, ie:
"psst, hey buddy, you need any perks? you can have 500 perks for $10."Some people will always look for shortcuts. Players rack up perks and achievements in EW/MW arenas (by killing squaddies or other "shady" characters) and use those perks to fly perk planes in Late War. If they can score a ton of perks the easy way, you can be sure it will happen.
Another worst case scenario to imagine: a handful of leet players from the past come back for a week of clubbing seals. You invite them into your squad with a generous perk bouquet. Then you have a whole squad of 262s running amok, until they get bored and quit again. Meantime the great unwashed masses (regular paying customers) are kind of pissed off.