When I own a card, its Gold number and the quantity I own are indicated.
If the quantity (the second number, after the "x") is greater than 1, then the card is available for trade.
I'm looking in priority for cards I don't own at all, whatever Gold value on the back. But I'm willing to get also cards I already have but with a different Gold value.
Also, all cards don't have the same back, but it's not my priority to get all cards with all backs!
For the differents backs, see
Ref. 2-01 : WA/DW - The Grand alliance
Ref. 2-01 : WA/DW - The hordes of darkness
Ref. 2-04 : BD-Imon, the mountain that breathes
Ref. 2-03 : HW - Nazall the nasty
Ref. 2-02 : SM - Hunt master Ethlich
Ref. 2-06 : LC - The brides of Styrkaar
Ref. 2-05 : WA/BD - Bearers of the sacred flame
Ref. 2-07 : BR - Twilight daughter of the blade