Thanks for considering purchasing our Samsung Galaxy Tab 7.0+! I can definitely answer those questions for you.
1) Retail stores or retailer means the store is a physical store and not on the internet.
2) None of our retail stores or authorized T-Mobile retailers use PayPal or anything similar.
3) If you friend purchases the Tablet at a T-Mobile retail store / authorized retailer for full price and completes the SIM unlock, you will be able to use the product in other countries.
It's also very important to check which frequencies your carrier runs their 3g network on. T-Mobile uses the 1700/2100 MHz frequencies for our 3g network. If the 3g network is on different frequencies, you won't be able to get 3g signal.
Hope this helps!