What type of transfer? Dye sublimation? Or just using heat transfer paper (great for personal use, horrible as a product you sell).
DTG or DTV are good professional options but they require expensive specialized equipment.
Dye sublimation is amazing but only works on polyester.
https://www.irs.gov/businesses/small-businesses-self-employed/simplified-option-for-home-office-deduction
Generally speaking, to take advantage of the home office deduction, the space needs to be used exclusively for business. Setting up a desk in a spare bedroom with a bed in it wouldn’t qualify you to claim the entire bedroom, but you could claim the space your desk occupies if the desk wasn’t used for personal use.