You're taking the typical New-Guy-On-The-Block path and photographing generic things around you. One third of your portfolio is images from a baseball game. They don't sell well to begin with.
Your remaining images are common landscape subjects. Look for equivalents in the database, specifically the ones that sell well. You will see how the others pop compared to yours.
This image is good:
But it's a common type of scene in the database.
I would suggest browsing through the database and looking at images that sell well. You will find the successful images are geared toward COMMERCIALISM and COMMUNICATE a specific type of CONCEPT. Or they're just head and shoulders above the rest.
People don't come here to buy YOUR pictures. They come here to buy pictures they NEED.
Try to cater toward THEIR needs. Sky's the limit in terms of subjects. You're better off experimenting with topics for which there is little competition. If you're going to do common subjects then make your images pop better than the rest.
Nikon D800, D100, Canon G15