This thought came to mind in light of this ability to sell the bonds for in game currency.

Maybe the best way to counter this it to reward loyal subscribers with funds to purchase items from the cash shop. By this I mean something as simple as a 10 dollar fund that can only be used in the cash shop and you obtain as a loyalty reward for every 6 months subscribed.

So basically you get 20 a year, 5 short of buying mount, enough to buy 1 plat bond or 2 gold bonds, enough to buy 2 phials or purchase 20 dyes.

Heck it even be just the right number to purchase all 4 minions.

I think it could be a potential happy compromise, leave the bonds sellable as is but allowing a loyalty benefit for time subbed would add incentive to stay subbed and feel like they aren't having to spend extra cash to obtain the cash shop items.