Consumer Price Index (CPI) as a variable in calculating gain or loss

I am in the process of downsizing and therefore selling off a lot of stuff I bought decades ago when a dollar was worth much more than today. There are inflation calculators online that allow me to figure out how much something I bought for $100 in 1960 would cost today ($939.01). Thus, if I sell it today for $900 that's a loss, right? 

So when I get a 1099-K from eBay citing that $900 sale, how do I use TurboTax DeLuxe to report it as a loss?