It’s not a bad start, by restricting trades to mediocre items, their value will lessen the incentive to buy them with real money. At the same time, when you won’t be able to trade legendaries this comes at the cost of frustration, when you search for a specific legendary for ages, and only find ones that you don’t need.
Apart from that one loophole concerning RMT is obviously that you can buy gold for RMT, by placing a common item in the bazaar for 2 million gold, and the gold seller buys that from you, giving you the gold that way.
It’s a slippery slope, and when in doubt, I’ll always tend towards freedom of trade, and whoever thinks they need to buy stuff for RMT shall do so, I won’t.
(Depending if all legendaries will have the same rarity, you could include a legendary for legendary trade. You can’t buy a legendary for gold, but if you have one, you can offer it against another legendary. Maybe even restrict each legendary to only be tradeable once, so that it leaves the economy as soon at it was traded.)