Excellent Marco. That's why I mentioned that you should really research before you bring home a ferret. I would also recommend everyone to read Kim Schillings "Ferrets for Dummies"....whether you are just considering a ferret or already have one. Many call it the Ferret Bible. It is chalk full of useful information and just about everything you need to know and what to expect from sharing your life with a carpet shark.
Pet stores indeed do not know enough about ferrets. I find myself on many occasions going in there and explaining how to do things or what not to do.
Ferrets can be a very expensive pet. They need regular things such as rabies and canine distemper, flea treatments....normal things that all animals need. Their food can be pricey, and sometimes they get sick. Some surgeries cost upwards of $1000 or more. We have vets in our area who treat ferrets, but there are some smaller towns that don't have the means to do so, so that just adds to your expense.