I'm VERY sorry to hear about your loss Tim. We have 3 Beardies ourselves and I know how we'd feel if we lost any of them.
It's hard to say who's to blame for what happened. I know it's a very bitter pill to swallow, but if your pet shop did not have this pet for very long in their possession before you bought him, it's entirely possible he was alread at least partially impacted when you brought him home. One thing I make sure of is that I feed my dragons a healthy vareity of greens to go along with the crickets. I know you did as well, but if the previous owner didn't feed him a balanced diet, that would definitely cause an impaction.
As was stated earlier, if he was impacted with Aspen Chips, you would definitely be able to discern them in what he vomited, so take heart that it doesn't appear to be the case here. From the sounds of your description of the mess, it sounds like he may have been housed with sand as a bedding. Sand has a very nasty habit of acting like sandpaper in a Beardie's digestive tract and that could definitely contribute to the gory mess you came home to.
I keep my Beardies on Repti-Carpet as well too. I absolutely love that stuff and swear by it. it's easy to clean and there is no chance my Beardies can get impacted from ingesting shavings, chips, sand or anything else.
I would let your pet shop know exactly what you came home to and maybe sit down and talk with them and let them know that in the future, unless you can verify the origins of the pets they get in, you simply won't purchase from them anymore. If they realize how important it is to verify the previous helath and origins of their pets, they may go that extra step to make sure the animals they sell are healthy and ready to be re-sold. it's just a suggestion.
Again, I share in the grief of your loss.