Having had no prior knowledge of the market for his product sweater, what approach did Jack McCarthy of Ugly Christmas Sweaters utilize when putting his product up for sale?

When any company wants to sale their product without prior knowledge of the market then it's a disaster.

First thing Jack can do is "Test Marketing". If any company has no knowledge of market then fastest way to know your market is test marketing. Jack must put his sweaters in small quantity for sale at different locations. Test marketing needs to be done for whole week. Jack needs to collect the feedback from its customers too. Feedback is the way jack can know the market. Without knowing the market and customers, it's really hard to market or sale your product. Feedback from test marketing activity will actually give him market knowledge and customer's feedback.