How do people sell steam wallet?

Wallet cards come with codes with this format : XXXXX-XXXXX-XXXXX. They can be traded just like game serials. I assume that's what you've seen in trades ?

I would avoid this. It's usually a scam, and if it's not, it's someone who doesn't know what's doing on. Now, they might mean that they'll buy you something worth $2 on their steam store, but I'd avoid people trading for wallet funds because that's inherently a scam.

There is, however, one method of doing this (they give you a skin, you list it so that you get $2, they buy it) but this is a very lossy transaction because of fees and Paypal is more popular.

You buy a csgo skin for 3¢ and put it on the market for you desired price and then they buy it. This isn't really recommended because they have to pay extra because of steams cut of the purchases