Writing on your skin with sharpie is not recommended.
When you write on your skin with sharpie the sharpies ink chemicals can leech into your skin and poison your blood which could make you sick.
Not everyone will get ink poisoning from the sharpie ink chemicals but some people have.
The sharpie marker ink will eventually wash off your skin and fade away but it can take awhile.
I used to use sharpie and other markers to write on my skin as a kid and teenager and nothing bad happened but some people are more allergic to certain chemicals than others so there's a possibility that you could get sick.
It's not a good thing to write on your skin with sharpie but you're unlikely to die from simply writing on your skin with sharpie.