Here are some players and methods I used to gain over 70k profit during the 6hour trial on the xbox one.

Now for some reason I can't access the web app due to me being a early access customer.. stupid I know.

You need to be prepared to take small profits.
You will not be a millionaire overnight (Unless you pack Cristiano etc..)
Dont give up on a player f he doesn't sell first time!
Always be prepared to sometimes make a few hundred coin loss' on tryin to find the right players.
For for gold players with ratings in between 80 - 84 and have good all round stats.

example player I traded with : Claudio Bravo - GK - Barcelona

Search him for a max buy now of around 2,000 coins and bid upto 2,000 coins on active auctions.

When you win the player put him straight up for 200 coins more than you bought him for.

example; Start price 2,000 - Buy it now 2,200

Like I said you will not be a millionaire over night.
Once you get a good amount if coins you can move up to higher rated players.

For example: Edinson Cavani - ST - PSG
Thr max I bid/bin for his was 8,500 coins.
I was winning lots of him and selling for 8,800 Buy it now.

The more knowledge of fifa player ratings and the demand of players will help you massively.

don't always go for the same players and be certain to find the right player to trade with that fits into you're style of trading.

Some players I have traded with are as follows;

Jesus Navas
Edinson Cavani
de Bruyne
Renee Adler
Carlos Tevez
Santi Cazorla

when trading the prices will vary and since my night of trading I Guarantee that my players prices I have used have changed.

This is not the best guide ever and I was not intended for that purpose, This is small paragraph or two on being a good trader and using you're own knowledge.