Is there a way to sell a game when u used the code and u havent installed the game?

is not forever, you can tranfer on other account by contacting steam support but you can't if you are vac-warned.

ps. you can transfer all but not only one.

However, if its on origin, origin is willing to transfer games from one account to the other providing you go through their process.

Its fairly painless, but pretty long since they want to make sure everything is legit.

you made me lol @ spider-man rule.

Thanks for the information. Just a heads up, though.

Distance Selling Regulations 2000, clause 8, allows you 7 working days to request a refund from Steam. I believe Steam products do not fall under "sealed product" and as such classified as a service and falls under this:

Source

for services – the consequence of agreeing to a service starting before the end of the usual seven working day cancellation period. See paragraph 3.22 under ‘Cancellation rights'

Exceptions to the right to cancel13.—(1) Unless the parties have agreed otherwise, the consumer will not have the right to cancel the contract by giving notice of cancellation pursuant to regulation 10 in respect of contracts—
(d)for the supply of audio or video recordings or computer software if they are unsealed by the consumer;

So it would depend if you consider once you accept the SSA as 'opening' the product. Though since you're only 'licensing' the product, I'm not sure if even that applies in this scenario?