Found a listing on eBay for Turbo Tax deluxe for $31.99. Is this legitimate?

Beware of deeply discounted prices for tax software online from sellers you do not know.  You can certainly purchase desktop software from a retailer for a lower price than you will find on the TT site, but purchase from reputable, familiar retailers like Amazon, Costco, Staples, BestBuy, Walmart, etc.  Buying a program from someone on eBay who will not be providing the license for you to download is a foolish  and risky choice.

Legal or not, it could not be used without a license code.  So there is no reason to purchase this item.

Starting this year 2019 TurboTax requires a valid license key in order to use.    

Only well-known reputable authorized resellers like Amazon, Staples, Office Depot, Newegg, Costco, etc. can sell TurboTax downloads.

Prices for Deluxe also depend on whether it has a state program included or not.   Current prices on Amazon, for instance, for the 2019 Deluxe are:

29.88 (Deluxe without state)
39.86 (Deluxe with state).