Gifting wars can be a friendly game between people, but they are often used to scam. Doing gifting wars with people who you don't know is very risky, and it's likely that they set up their gifting war with the purpose of scamming. Even buddies can scam using gifting wars, since anyone can pretend to be a friend (or even a close friend) for rares, and suddenly break someone's trust.
In gifting wars, one person gifts someone a rare item through Jam-A-Grams (Jags), and the other person must gift a rarer item back. This usually continues for a while. Scammers block the person they're playing with once they get a significantly rarer item than one they just sent. Sometimes, a scammer will go to a public place and say "Jag me an item to start a gifting war!", but the scammer never gifts anything back at all.
Since nonmembers are unable to gift, the nonmember equivalent to gifting wars are trading wars. Trading wars have the same exact concept as gifting wars, except that instead of gifting, players trade good items for the other person's bad items. Trading wars can just as easily be used to scam.
How to avoid this scam:
The only way to ensure that you won't get scammed by gifting/trading wars is to not do gifting and trading wars. If you want to do them, make sure it's with a close friend. Even then, a close friend can still scam you, even if you think they'd never break your trust. If someone you don't know well wants you to do a gifting or trading war, it's more than likely that their only purpose is to scam you.