I think Marte would be a great addition to your minor league roster (I added him to mine last year). While he gets over-shadowed by Wilmer Flores, some scouts say he has a better chance of staying at 3B because he's not a complete butcher, and Flores may end up moving to another position.
Here's a brief synopsis on Marte from last year at MetsProspectus:
"Scouting Report: If you're going to talk about upside, you have to talk about 3B Jefry Marte. No, he's no Wilmer Flores, but he isn't far from it. Marte, like Flores, was signed last year as an international free agent at age of 16. The Mets put the 17 year old Marte in the GCL to start his pro career, and since joining the team has done nothing but hit for power. In 18 games he's put up an incredible line of .328/.408/.639/1.048. That high slugging and OPS are in result from the 12 XBH (3 HR, 8 2B, 1 3B) he has in just 61 AB. If anyone is going to fly up this prospect list it's going to be Marte."
Also, if you're looking for other up-and-coming young guys, here a few who may or may not be available in your league to keep an eye on:
Jharmidy De Jesus-3B/Sea