Thank you for your question. I look forward to working with you to provide you the information you are seeking.
The wonderful thing about capitalism in the US, is you can open up a competitor store right next door to this guy and as long as you are not violating his agreements with T-Mobile to sell T-Mobile products or services, there is nothing he can sue you for unless the landlord who you are renting from is also renting to him and you two have an exclusivity or non-compete agreement in your lease
which prevents you from selling the same or substantially similar products and in that case he could stop you from selling any cell phones or cell services.
You need to start by checking your lease for any restrictions. If your lease has no restrictions and you are not violating any of T-Mobile's rights, he can sue you but it would be dismissed because there is no ground for him to sue you under.
There are no legal documents you can get to avoid problems with him. All you can do is send him a letter informing him you are not violating any terms of your lease or any T-Mobile rights and as such you have the right to sell whatever you please.
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