G2A and Steam Terms of Service

You should read the terms of service for yourself so you dont base something important on a strangers opinion , in other thought even if it does i doupt they can find out if you get money when you are trading a key unless someone calls you out

What are you concerned about? Getting banned from Steam for selling gifts? They want your continued business, I don't see how it would be in their interest to give you the boot.

Send me a link to the ToS, and tell me which clauses to look at, I'll analyze different interpretations for you

I doubt anyone is selling "Steam Gifts" on G2A. As for the resold keys, you'll need to check the ToS from wherever they came from. It probably has nothing to do with Valve.

Generally, once a manufacturer or store has sold you something that is the end of their "influence" on the product. They can't stop you from selling it on somewhere else on the internet or in real life in a pub etc. The same applies to to manufacturers selling stuff to retailers. They can not stop the retailer from selling it on to other ones. Imagine you could only buy a car from manfacturer-approved dealers. We would end up throwing away the last 30/40 years of progress for consumer rights, especially lower prices.
In the 80s and 90s car manufacturers tried to stop cars being sold to people in other countries for lower prices by all kinds of methods. These illegal practices by the manufacturers were subsequently stopped by the courts.