The latest Bitcoin miner on the market is currently the Antminer S17. At 53TH/s this Antminer is the fastest yet, but not the exponential increase many had been expecting.
For the time being this guide will focus on the trusty Antminer S9 series. Ebay or Amazon is the best way to go about your purchase.
Use all the listings on Amazon and Ebay to get an understanding of the current average market value. On Ebay focus on buy it now listings. Pay attention to whether or not a power supply is included. If it is not included Parallel Mining is your best bet for a newly crafted PSU available for 110 or 220. Find something for a fair price based on market values. No reason not to buy new at current market prices as of 12/7/2018.
Choosing a Power Supply (110 or 220)
Setting up a Bitcoin Mine