Depends on the brand.
You need to register to be an authorized apple reseller as well as a dell partner, but only for new machines.
2nd hand parts and 2nd hand equipment are usually fair game.
However your country and state laws might have some weird nuisances.
And if you dont have the right contract agreement in place when you sell it, there might be issues with customers returning the product.
Look at your local governments website and download the PDF like in my country regarding specific issues of that nature.
Be careful as well you don't start a "sole trader" business instead of a company and make all your previous equipment liable for litigation if someone goes at you through the law.
even your dirty socks can be sold off to pay the debt if you take that creaky road.
If you are a real company and not a sole trader, the definition of your company will allow you to buy parts wholesale in multiple units and you can pass on the costs of import tax etc plus GST.
depends on our countries laws and requirements at the end of the day. You should have your States Gov website bookmarked or at least the whole website cached on your system.