Its not Open Bazaar's fault or anyone's fault. Its the deflationary nature of BTC thats the culprit. It makes BTC treated more like "digital gold" than a currency. So holders arent very willing to part with their BTC as easily as they part with their fiat.
That is exactly my position when I read about KFC implementing the use of bitcoin and accepting it for payment because people in the crypto world are only seeing bitcoin as a store of value and they would prefer to convert to fiat, meet their need than to pay directly with bitcoin because as humans you get the feeling that the you would have gained more if you had not spend this money on that pizza or that chicken and the rice or burger that KFC is offering. The same thing is applicable to open Bazar a nice business that wants to make things available for every one but the market is not yet ready for it.
I would not say its a failure rather I would say the majority of the market participants have their attention into somewhere else and at the time they are ready, the business will be one that we would come to recognise as an ingenious development.