firstly, you must have enough funds in your steam wallet, and your account must be validated. I think you can have it validated when you purchase or add funds in your steam wallet and then they will send an e-mail for you to authorize the transaction.

However, the next requirement is a bummer. For you to purchase anything from the community market, you have to wait at least a month from your first transaction. I think this is to somewhat prevent scammers from creatug dupe account and buying stuff from the market.