Add popular items on the Market Board to your wishlist (such as Vanya- or various housing craft items and minions) and wait for somebody to muck up the sell price (like forgetting a 0 or trying to undercut other sellers by a ridiculously broad margin), then buy it and sell for a profit!

Of course though this occurs very rarely.

Farming shards and crystals in demand can earn you a quite a fortune if you can dedicate a (massive) portion of your time online to gathering them on a daily basis. Be sure to check the Market Board daily, given the prices tend to fluctuate every couple of days.