Unless you're trying to sell an Ld or a Cr, I can't think of a problem..
Kate buys cards for like 0.1 % of their value thats what he means although if you are a free player i dont see you have any valuable cards
Just try doing a sponsor pay offer to get rid of the market ban. The 2 credit ones only require you to put in (and sometimes) confirm an e-mail. Plus you get credits, so it's a win-win situation.
Just do a survey for credits. You both get credits (win) and the ability to sell on the market (win.) Only takes about 15 minutes, too.
If your saying you can't bring yourself around to click accept because you won't get much then don't lol. Waite intill you can buy credits (you only need to do it once to unlock the market).
If its that the game says you can't then it might be a Ld card which can't be sold to kate (or anyone).