You need to change the condition of your modern cards from mint to nrmt, that's what the online price guide bases their pricing on. If you do that, then you'll be on the same page as most every other trader on here. All cards marked as mint, are priced at a premium, so it throws off the trading values.
Collect Andrew McCutchen, Roberto Clemente and most current/vintage Pirates. Some Braves Dynasty like Maddux, Klesko, Javy & Chipper.