A buyers auction is a way for you to buy something without having to do a lot of work yourself. You basically list what you want to buy and in most cases a general idea of what you're willing to spend, any specifics you want to include and then you sit back and let the sellers compete for your business.
You'll get connected selling auction linked directly to your Buyers Auction and you can either bid on one or more of them or even disconnect the auction from yours if you have no interest.
Sellers can see what other sellers are offering for your consideration and make adjustments to pricing or other methods to entice you to purchase theirs over the competition. In the end, you should get the best deal possible since sellers are actively competing to earn your business.