My sister has a t-mobile family plan with 5 lines. Actually they don't use that much. So I will share one line with her. I want to buy a used t-mobile phone online. Can I use it by just putting the sim card into the cellphone?

I also have some concern about the source of the cellphone. Can it be lost ones or stolen ones then sell to me? If I am so unlucky to get such one, can I use it? Can I check with t-mobile with the cellphone's IMEI number? I am not sure when you say something like locked is it the IMEI number locked with the former phone number?

Thanks!