First, I'll say that I've helped fellow collectors before, including selling extras that I have at below market value.
My collection is focused on official minor league baseballs. I have withheld info from an ebay seller before. An auction listed two Little League baseballs with a starting bid of $1.50. They were described as one Little League baseball and one Pony League baseball. The assumption was that the Pony League ball was from the community baseball league for 13- to 14-year-olds.
I took a closer look at the pictures and realized it was a 1940s-era PONY League ball, as in Pennsylvania-Ontario-New York League, an affiliated minor league that was the precursor to the modern-day New York-Penn League.
I considered informing the seller for about a second, then decided against it. I did win the auction (the ball can be seen
here).
In a weird coincidence, somebody else put another one of these balls up for sale about a month later, who clearly knew what they had. It sold for $150.
www.minorleaguebaseballs.com