Well, I don't think you should listen to what PETCO says, because budgies are not supposed to live on a carpet behind glass, as you know. I'm glad you posted the picture of how it is there, though.
The messy rump can be a symptom of pretty much anything. You can give him a couple of days with low stress, enough food and water etc, but you might want to give him a real check up. It might harm the taming process, so I think you should try to do as I mentioned above, because it might just be stress from the shop.
It looks like a hen to me
Anyway, he/she has such a lovely colour! In case you don't already know, it's fully grown up and might be hard to tame. He/she is also an opaline, but I'm not good with colours.
If you take him back to the shop, they won't get the bird help, they'll just adopt him/her to someone who doesn't notice the rump.
I hope everything will work out fine for you.