Do you post the same design on multiple websites?
Also, how come some users post things like BMW logo or whatever. Are they allowed to do this?
how about copyright infringement? can i just use footage of a famouse tv show`?
Most POD sites, including Merch by Amazon, are non-exclusive so, yes, you can post the same design on multiple sites.
No, you cannot post clips, images or logos of movies, tv shows, sports teams, colleges, etc.
Sometimes a line or catch phrase is okay but you need to make sure it's not copyright.
Even lines not from an intellectual property can be copyright.
For example: "What's your super power?" is trademarked for shirts. You can't make any merch with lines such as "I'm a Beagle mom. What's your super power?" or "I play clarinet. What's your super power?"
A lot of people violate trademarks but most sites, especially Amazon, delete those listings and ban the account.
Merch Ninja (paid tool) lists all shirt designs that have been deleted by Amazon. You would be surprised what is flagged.
Note that redbubble has agreements with some rights holders and you can create "fan art" for those properties. Rock band KISS and movie The Big Lebowski (and others) can be used for merch on redbubble, for example.