The problem is that if they sold you said poorly hamster and something happened to it (like it died - worst case senario) then you'd be back to complain like any decent human being would.
They can't knowingly sell a poorly hammie - or any other animal.
Sadly the majority of the stores are hugely under staffed, the person on check out is not allowed to leave the tills (although I have been known to wonder round to the small animals on particularly quiet days but I could loose my job for this) and other staff would be working delivery or taking care of the animals out the back which would be in Quarantine or Isolation.
If you have any problems like this I would ask to speak to the Duty Manager, this will either be the Store Manage, the Deputy or Assistant. They HAVE to take your concerns seriously and get back to you with an up-date, in this case let you know in a couple of days if the hamster is well enough to be sold to you.
If you don't get any joy then I would take the matter further ie contact Customer Services or Head Office.
In the last 2 months I have taken a total of 5 russians off shop floor to go into Isolation due to (minor) injuries caused by cage-mates, they spend 1-2 days on their own and have all been okay
Let us know what happens