Selling Steam gifts through E-Bay or similar sites (is it legal)?

And why you don't use Steamtrades?

It's against Steam's TOS.

read the EULA / TOS yourself

People do it all the time. They buy it on sale and post the starting bid a few cents higher than that. Not saying it follows steams rules though.

Although people do it all the time.
The sale of digital goods is against EBay rules.
Hence why many people state that the code will be physically posted as this is the only way EBay will not take action against the sellers.
Paypal users can simply file a dispute and automatically gain their money back as EBay and Paypal will simply refund the buyer due to the fact that the goods are digital.