A man in the middle is (as said) not feasible.
There are reasons why I came up with the no-support rule, specially with an eye at the scams over the last time.
I.E. if you pay with USD but seller don't sent tc/move tc/duel with that tc or whatsoever it ends up with generating new tc in case that seller tc is gone by duels etc. (tbh we cannot strip that from other users if they won it at a legal duel from their side of view)
Getting proof of external real money transactions is also not always that easy. With i.E. webmoney terminal and similar you don't have always hard proof to show up (unless you take a pic with mobile of it
)
Anyway, you are still able to buy money from the shop (also never forget that u support the game with it!) beside lots of other options to make tc (boosters, tourneys, marketing, art etc.), so even beginners are able to collect tc w/o buying them externally.
Take a look at other games which have some kind of transferable ingame currencies. Most of that companies don't support external sales or completly disallow it with strict rules and a complete ban if you get catched.
Second Life in example also don't give any support on sales/buyings outside of their own shop:
5.3 There are other Linden dollar exchanges that are operated by third parties.
Linden dollars are occasionally Bought and Sold on third party exchanges, which are wholly distinct from the LindeX exchange. Unless specified by Linden Lab, these exchanges have no affiliation with Linden Lab. We do not guarantee the legitimacy of the Linden dollar transfers offered on them, and we are not liable for purchases of such Linden dollars. You purchase Linden dollars from such sites at your own risk. If you Buy Linden dollars that are traced to unauthorized credit card activity or other fraudulent activity, we will recoup these Linden dollars from your Account.
or World of Warcraft as 2nd example:
Ownership/Selling of the Account or Virtual Items.
Blizzard does not recognize the transfer of World of Warcraft Accounts or BNET Accounts (each an “Account”). You may not purchase, sell, gift or trade any Account, or offer to purchase, sell, gift or trade any Account, and any such attempt shall be null and void. Blizzard owns, has licensed, or otherwise has rights to all of the content that appears in the Game. You agree that you have no right or title in or to any such content, including without limitation the virtual goods or currency appearing or originating in the Game, or any other attributes associated with any Account. Blizzard does not recognize any purported transfers of virtual property executed outside of the Game, or the purported sale, gift or trade in the “real world” of anything that appears or originates in the Game. Accordingly, you may not sell in-game items or currency for “real” money, or exchange those items or currency for value outside of the Game.
..and there are many more with similar restrictions for their virtual currencies.