@Eliatack
Good question! There are so many opinions on this topic, and I think the most successful pens tend to be blue ballpoint, not sharpie. Displaying your ball in a uv protected ball cube (inexpensive at your local card shop or on amazon) not in direct sunlight are key. Air and light are your worst enemies for signed baseballs. Also, getting signatures on actual leather baseballs (as I am sure your minor league ball is) is the key, as the cheap baseballs from Walmart tend to fade really quickly.
Cheers!
Kevin