Just a few pointers:
1. In this game, the trading currency is typically Time Elixir (TE) or Potion (Pot); and a trade is considered pure if you only pay in TE and/or Pot. Luna is generally not accepted because luna is easy to get and most people have more luna than they need already...
2. At the moment, the cheapest acceptable 6* goes around 6-8 TE. (i.e. Basically all 6* is considered to be acceptable except for Alexander and Mina)
3. Most people don't buy 5* unless it is a Limited Edition (LE) or max level. LE 5* is mainly for collection, and max level 5* is mainly being used as enhancing feeder
4. Almost no one will down trade a 6* for a bunch of 5*s. So your best bet is probably to max those 5*, sell them at 2-3 TE each, then use the TE you gained to buy the 6* you want. Though, TE/Pot shouldn't really be a problem basically many of events have been giving them out recently
5. MLB stands for maxed level break. Level Break is a process where you use ambrosia to increase the maximum level of the card. For a 6*, you can level break it from 120 to 140; and a MLB 6* basically is a 6* that is at 140 level