advise needed for small scale builder

i am a very small scale repair service provider who also from time to time builds a full system for a repair customer. to date around 10 to 12 a year.

my ? is can i legally use oem software purchased from companys like new egg or tiger direct when i sell i complete system ” i’m to small to purchase from main line like tech data” or should i continue to use full retail copys to stay out of problems with microsoft with all the new laws,

my main line is repair and refurbish for which i have always used retail copies to upgrade the used units. i worry big time as i’m handicaped from a spine injury and really don’t have funds to legally defend my self if needed. but still need this work to help keep active.