I am just frustrated because there's this neat little gadget that I covet, and I look at getting one and find out it will be nearly $120 to my doorstep. I think that's a little excessive. I'm not suggesting that it should be $6 plus a little margin, and I think it's entirely reasonable for the developer to make themselves something of a living from the device.
I just think that $120 on $6 of parts is a little excessive.
But then, I live in a very different business environment to the EU. Sales tax here is 8.25%, and doesn't apply to any out of state order. There's no state income tax. I ship things to the EU and postage is less than when you do. Also, my LLC gets to pay almost zero taxes as long as turnover <$1m. I should remember businesses in Europe face a very caustic trading environment.

The other thing I need to remember is that I am not trying to support myself with this venture. I am giving my time for free and charging what amounts to a 6.5% markup on what I sell, so there's enough to cover losses, and a little left for the next project.
Last word: my ire was aimed at the manufacturer, not you. I have never thought you were ripping anyone off. I think the guy who designed and makes them is just doing a little better than is necessary - he could make more money selling a higher volume on a slightly lower price. A 10:1 mark-up instead of a 20:1 markup would be nice. I imagine he leaves quite a small amount of meat on the bone for resellers like yourself - not that that is a public matter or any of my business.