I have seen a few of your threads and i am gutted for you over the loss of little Stanley
As others have said dont beat yourself up or blame yourself in any way for how he died, i dont want to upset you any more than you already are but i was looking at your pictures of him and i honestly dont think he looked healthy at all, for a reputable shop to sell you a stick thin beardie (as gorgeous as he was) with wood chip substrate they are the only people to blame in my eyes.
I can only advise if you are to get another beardie contact a breeder nearby who is dedicated to them and will give them the best possible start in life before he/she gets to you, dont give up on them over this mishap, i have lost one before and when i think back it looked ill when i bought it but as humans we try our best.
As for the vet pffffffffft!! im no expert but you just dont put a baby beardie on sand, best going with vinyl tiles, lino, kitchen towel etc
Once again sorry for your loss, keep your chin up