Should I buy an iPhone 6/6S off of eBay?

Some auctions on there has iPhones for $0.99, and I just wanted to make sure IIm not getting scammed. Also, how do you pay the phone bills if you buy a used iPhone off of eBay?

You're taking a risk of buying someone else's problem. Any iPhone that costs $0.99 is more than likely broken and not fixable. You can buy them (you'd only be out $0.99 and whatever shipping was), but they aren't going to work. Unfortunately, unless the iPhone 6/6s price on eBay is somewhat near what they go for new right now, it's probably junk.

Sure

First of, it's quite unlikely that you will get an iPhone for 99cents. The auction starts at that price so that people are more likely to bid on it. In the end, it might cost well over $400 depending on the condition of the phone, the model, capacity, etc…

Next, you have to sign up with a carrier to get service. Which carrier depends on the phone. If it's locked to a specific carrier, you'll need to use that carrier. For example, if you buy a T-Mobile iPhone, you have to use T-Mobile. If the phone is unlocked, you can use it in pretty much all carriers around the world as long as they're compatible with the phone.

Next, make sure that the MEID/IMEI is not blacklisted. People often sell lost or stolen phones, which have their unique numbers blacklisted, preventing you from ever using it. Also, make sure that the phone isn't iCloud locked.

Like David said, you're taking a risk when buying a phone since you don't know the history behind that phone. There are reputable sellers who are honest and do their best to sell phones that work, while there are others who aren't honest. However, most purchases on eBay are covered through eBay's money back guarantee if the device doesn't arrive as it's described in the listing.

Like you say "auction". It may start off with the price of $0.99, but the final price is around the actual value. Read the seller reviews to prevent getting scammed.