Sure, that's not all that uncommon either, but it depends on the job, of course.
To use your shoe example: If you are in the professional athlete world or in the advertising industry, for example, what you sell on the side and what money you are allowed to take on the side is very strictly governed.
Same goes if you're a college coach -- I'm sure we won't be seeing Whittingham wearing Nikes anytime soon for a reason.
If you are just some accountant selling something on eBay, or receiving a gift from a friend, then yeah, that's probably not covered in your contract.
Not sure where you are going with this, BTW.