I have a huge database of UPC barcodes and their corresponding products (name, photo of the product, some specs).
Do you think there might be a market to provide a paid API access to this data?
A quick Google search reveals that such a product already exists: http://upcdatabase.org/api. Even worse (for you) it appears to be free, with the request of a donation if you're using it for commercial purposes.
I didn't look long. I just clicked on the first link. I'm sure there were others.
But the fact that others are doing it doesn't necessarily mean you can't do it too. Indeed, the fact that others are doing it is a good indication that there is a market, as per your original question. Just the raw fact that you'd have competition is not a deal breaker by any stretch.
You do have to look around and see what you can offer that isn't currently being well served. That site that I linked to seems to also have a large data set (1.5M records) , as well as photos (in some cases). The specs you mention might be a selling point, depending on what the specs are, and what existing services already provide.
Also, there are other ways to compete as well besides just more data. For example, you could compete on performance, simplicity of the API (or integrating the API), customer service, or on supporting features (like maybe a search that gives you UPC codes by tag/topic/keyword?).