Yes, there are significant restrictions and you can't take a game out of your library and gift it, for example. I believe there may be other restrictions on, for example, having people trade when one person is in a low price region and gifting to someone higher, but I'm not very familiar with the details.
These are not restrictions. At this point you DO NOT own a game yet, it is still packaged like an unopened box wrapped in plastic wrap. It is essentially the same as if you buy a new phone or dvd and without doing anything to the box simply resell it to a different person.
Toying with gifts the way you mention may get you banned, there are certain restrictions as well which may result in you not being able to launch a game since you are not in the specified region (ip check) and vpn/proxy use is against ToS so it is a gray area, you keep doing it and you get banned eventually.
When it comes to TF2 items each player has a choice whether he invests time or money to get the resources to craft items, i'm not an expert on this but since resources are made literally out of thin air there is mostly supply/demand.
You can claim it's a voucher instead of a game, but the distinction is quite meaningless to me. I can see why they do this for business reasons, but it's not something we tolerate elsewhere, generally. And why is it not a restriction if I am forbidden from doing something?
We can redefine all the words if you want, but the essential facts that I've paid them money for something and they forbid me from doing certain things with the item I purchased remain.
Do they? Only if you haven't claimed the game, I believe. You cannot sell (or trade) games that you have in your library.