Hi!
A white trade is a trade where one of the player already own the creature they are about te receive. They earn nothing with the trade while the other player earn a new creature, thus a white trade include often two creatures with the same rarity, to be sure it is a fair trade even for the player already owning the creature.
For example, I traded with you the August Lunaris Osiris. You didn't own this creature, but I already owned all your duplicates. Osiris is a 3 stars (3*) familiar creature, you gave me in exchange a random 3* familiar creature. Same rarity, it was a fair white trade
I hope it is clearer for you, do not hesitate to ask if it isn't yet.
For your future trades, a new trading system is to be released soon, it should be easier with it to know what to propose in a trade. But you can already look for the previous month creature, you could easily trade your last august creatures for july or june creatures. For the higher-value creature, people often prefer other rare creatures against them, but sometimes some generous players offer rare creatures against less rare ones (or even as gift in games or raffles). But don't worry, you can also obtain freely rare creatures in two big events per year, one being the Christmas Snowballs Battle (for the Christmas Holidays, of course) and the other the Gothi-Birthday's Pies Battle
I'm at your disposal if you want to know other things about Gothicat World