Why would someone pay you $15 for CSS when they can buy it themselves for $5? They'd just be paying an extra $10 for you to make the account for them (which is free), and forfeiting their ability to choose their own username/whatever-else-you-choose-when-registering.
You'd be better off waiting for TF2 to go on sale again for $2.50, then selling it at half-price when the price goes back up. I know someone who did that. Bought ~20 copies at $2.50 each.