You have answered your own question really - until you spend money on this game it only allows you to sell 1 character at Market every 24 hours.

It's a bonus for getting higher than a certain level. I don't remember the exact level though...
It's either, 10, 15, 20, or 30.... Go figure...

Actually, if you joined during a certain time span, not certain the exact dates you can sell 1 per 24 hours if you joined after the date you can't sell at all until you buy credits or do one of the nice free offters, the most likely reason is to stop multi abuse of credit and card farming as well as mission abuse. and if you joined before that certain date i believe it is the same can't sell till you get credits. hope that helps.

Wildthing answer it

in short period of time (around 6 months)
every player that join around that date can sell one card per day.

but after that UR close it and now, new player need to buy credits to unlock "sell to the market" feature (like the old days)

Iam not clear if i reach 46 can i sell caracters in markets too ?

If you joined after said date, then even if you reach level 46 you still can't sell on the market, but if you do one of the free offers on the main page of Urban Rivals then you can sell or if you buy actual credits that works too. hope that helps.

I did three of the free offers, received credits but still cant sell characters??

Players who joined back 1 year also cant sell 1 card per day,because i also joined 1 year before and couldnt sell till lvl 50 , so i buyed credits and now i cann sell in marke

@battlefire
Keep doing them until you get stars. Then it's unlocked for sure.