Know your seller.
Check feedback.
I have found that new products sold by ebay companies are rarely a good deal.
They need to make a profit.
If this is from a private seller, then possibly it is a good deal.
someone might have bought one and had second thoughts for skylake.
FWIW
With Skylake now available, there is absolutely no question in my mind that a new build should be Skylake.
a. Prices for cpu, z170 motherboard and ddr4 ram are almost precisely the same.
b. 6600K has an estimated 5-10% performance improvement per clock over haswell.
c. 14nm runs cooler, you get a decent overclock without the need for exotic cooling.
d. The Z170 chipset permits the use of much faster ssd devices on the horizon. Samsung 950 pro for example:
http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/html/ssd950pro/overview.html
For gaming, the cheaper i5-6600K will be as good as it gets and cheaper.
Ebay takes the buyer's side on transactions so you should be protected assuming you pay with paypal.