You can sell fractional shares. People do it with mutual funds all the time, with the number of shares calculated to 3 decimal places. There are also situations where fractional shares of stock are sold. You can certainly report a sale of 1.5 shares.
Note that the number of shares is not actually entered on Form 1099-B. There's no place for it on the form. The information is usually provided by the broker, and it's common to include it in the description on Form 8949, but the number of shares is not required. The figures that actually get reported are the dollar amounts of the proceeds and the basis, not the number of shares.
