Does your account unlock steam market ?
I don't know what this actually mean,you can use the community market and there are no restrictions on the account.
It means if you are able to sell in game items on market for cash. By default to enable this functionality you need to purchase at least one game via Steam. Dota2 alone won't be able to do so.
Oh, i didnt know, its true, it cant sell the items unless you buy something from the store, after that its working, thanks.
You can unlock market cheaply by buying some super deal, like this game that is currently for 0.14 euro
http://store.steampowered.com/app/96100/?snr=1_7_7_ut2_150_1 or simply sort by cheapest and you might find an even better deal. I bought that one for my alt accounts and it worked fine so that shouldnt be a problem for potential buyers.