Fuck Wit Dre Day

The Chronic Dr. Dre
Actually, "mark" is a derrogatory term. Dre is saying just by affiliation, he was a loser.
the only thing you are good for is making fun of you're penis, or lack thereof
In this song he is poking fun at then rival Easy-E's standard outfit of a baseball cap with the word Compton written across the front and style of dark sunglasses know as "locs". He is insinuating this is a "tough guy" costume for Easy and a facade.
In this case this is a dig at rapper Easy-E. Dre is saying that people thought of him as a "mark" (a term used to describe someone who is easy to take advantage of) because he used to be affiliated with Easy-E and this was bad for his reputation.
He was the target of negativity and/or guns because of his affiliation with rapper Eazy-E
I know you can't fight worth a darn, and this leads me to believe you must be carrying a gun with you in case a physical altercation would ensue.