A trade is never okay as such, since there are no trades in the BuzzerBeater market mechanism. What this means is that any trade between two teams is considered as two (or more) separate player transfers. Each player transfer must be fair and conducted at market price. If one (or more) transfer (in a player trade) fail to meet this requirement there will be consequences.
So, we will not specify how to trade properly, because there really is no trading. We do specify that each player transfer is considered as an individual transaction that needs to be fair. Since it is difficult to make sure two teams are able to fairly transfer specific players between themselves, it is not advised for anyone to get involved in it. Such activity is very suspicious each and every time, which is kind of proved by your questioning in this thread.
I would like to take this opportunity to remind everyone that you should report a suspicious transfer when you see one.