I thought of that one, but discard it due to
Remember that buying gold is based on a % of your IPH or Level or something like that, so I think only high level players would get "lots" of gold on a purchase, BUT
1- Any top player (or long time player) would have no need for it as they IPH would be high enough and they would save a few hundred million for EB (My IPH is not that great, but I have on hand 200m+ gold)
2- I think it would actually be cheaper to get gems and use 5 gems per hit, intead of buying gold and using gold hits
But yes, that is another potential source of revenues, same as buying gems to use gem hits on EB