1. Can the account you are selling get banned by Steam?
Yes, the account can get banned, but after 2-3 years of creating accounts and or switching regions none of the accounts were banned.
2. Do I need to use a VPN or any other anonymizer to use the account?
Nope! No VPNs required, just type in your username and password and you are good to go!
3. How can I buy games when I am not from these countries?
By purchasing Steam Gift Cards from many unnamed websites for the country the account is created in.
ex. Argentina: ARS$; Russia: RUB/₽; Turkey: TRY/TL/₺
4. Can I play these games even from my main account?
If Steam Family Sharing is enabled for the game then yes. But you can always play the games on your purchased account.
5. Can I get cheaper microtransactions?
Well yes but actually no.
Depends on the game, for example Warframe lately only Prime packs that are listed on the Steam store were cheaper. IN-GAME microtransactions weren't cheaper.