Now if I were writing an article for a magazine, yes of course I would let him read the piece and try to work it out with him

To me this would be inappropriate with both print and online writing (which should be treated the same). If you've written what you believe to be true, there's no excuse for changing it because any change will by definition make it something you don't believe to be true. And what does that make you? Every concern you cite -- "I feel bad for him," "I donīt want to ruin my relationship with him, because he could be helpful to me in the future" -- works against truth. Once you focus on truth, all your other decisions become easy.
There's nothing wrong with talking to people about being a blogger, but you've got to be able to look them in the eye and say "no" when they try to manipulate you. If the item is being offered for sale to the public, you should feel absolutely free to write about it. Had he taken you in the back and let you preview an item that was not yet for sale, that would perhaps be a different story -- you'd have to use your judgment regarding the value of writing about something that's in the development phase. But the shopkeeper's feelings, your relationship and what he can do for you in the future still would not be relevant considerations.
Most of us who write over a period of years have on occasion been tempted to allow sympathy or other improper considerations to cause us to alter what we write. I'm very grateful to those who have stopped me from making this mistake.
This is not the same as your situation, but maybe it will help as an illustration:
A few years ago, when I was working on my first book, I spent several days in a restaurant kitchen. I spent time in many restaurant kitchens, actually, but there was one in particular that I'm talking about here. The restaurant was boring. After days in the kitchen, I had seen nothing interesting -- nothing that I felt would be informative to my readers. Nonetheless, the chef had been generous with his time, and I felt I owed it to him to write something about the time I had spent in his kitchen. What I wrote was as boring as my time in that unremarkable kitchen. Everything I did to make it seem interesting seemed forced. But I felt I had an obligation to include that material in the book.
My sister, who is an editor at the Wall Street Journal, is one of the few people I let see my work before I show it to my real editor at the publishing company. She read these pages and said, "This is just not interesting. You should get rid of it." And I said, "But the chef was so generous with his time!" And she said, definitively, "Your only consideration should be whether this material helps make the book better. It doesn't. Get rid of it." She was right. I got rid of it. And I've carried her words with me, applying them in many different contexts, ever since.
So if you're saying "this piece really belongs in my series," I think you have your answer.
P.S. Yes, the chef was mad at me. But not as mad as I'd have been at myself if I'd done the wrong thing.