One piece of art can reach several kinds of buyers. Someone who cannot afford an original may happily buy a print or a shirt.
Originals, prints, and merch can all come from the same piece. The key is keeping a simple list of what you have sold in each form, so you never promise the same thing twice.
Decide which pieces will be one-of-a-kind and which can be reproduced.
Keep a list of every sale: the piece, the form, the date, and the price.
Tell buyers clearly what they are getting.
Next step: see how we present artwork in the shop.