Sorry for your losses. Always difficult.
Any red sores noticed on the first fish? I'm assuming too hard to tell on the second due to the crabs getting to it. I ask because I think disease is a definite possibility here and am wondering about uronema which is a particular problem with clowns. Unfortunately, it also has no fallow period.
Anyway, hopefully it's not uronema and going fallow will clear up your DT. I'm also in the QT camp and would recommend doing that from now on, but it sounds like you've jumped on that train on your own. I also do some preventative meds in QT, but that is a big debate and I'd recommend you read up on that in the QT sticky threads. In any case, I'd suggest that you start with a "cheap" fish that you still want to keep long term to make sure your setup is hospitable. However, I know that some of those sexy designer clowns are hard to resist!
So sorry for the losses. Unfortunately some fish stores are great and others are terrible. I wouldn't say we can rule that the store is terrible because 2 of the fish they sold you died but it would be good to try someone else out for a bit and give the store a chance again sometime in the future. I am willing to drive 2 hours to buy a fish because of the great disparity of fish store quality.