Sellers with thousands of items at low prices

I have listed a few items and bought maybe 10 over the past two years. Can someone explain to me how a seller can have 15k listings and also how they can sell brand new items so cheap? For example, I just bought a pair of beautiful Paul Green boots that would normally cost $500 for $70. I thought this was going to be a scam but risked it anyway. Just got them and they are gorgeous and look brand new. How is this possible? The seller says they are in the U.S. and has other low-priced items.