He had a used pinball machine for sale, which he advertised in the classifieds of the August 11 issue of the Hill Valley Telegraph. According to the ad, the machine didn't work, but would be "great for parts".
The pinball machine cost $50.00, and was purchased by Emmett Brown, who used the parts to create a fake nuclear weapon for the Libyan terrorists, while appropriating their plutonium for a less harmful project, i.e. to build his DeLorean time machine. Doc had already bought a DeLorean DMC-12 for this purpose from another Hill Valley resident named Robert.