I did a radon test that scored 18, but the cannister pick up date and time was not marked, the buyer picked it up for me and mailed it in; without marking the time and day. So I did another radon test and this time the seller removed the bucket the cannister was setting on. My question is: Will this radon test be useable or accurate?

Nope. But if you informed the seller that the canister could not be moved, in this case, you could recommend that he pay for the third test.

Mistake 1: using canisters in stead of a CRM
Mistake 2: allowing the buyer to pick-up canisters.
Mistake 3: not educating seller of proper protocols (possibly)

Absolutely correct Mark. Why would you be having the customer picking this stuff up for you in the first place, isn’t that your job as the inspector?

Only time I have ever done that was when the test was being done for the home owner themselves, and it wasn’t in regards to a RE transaction.