I have a Ditto, too, actually...
Anyway, I'm here because I've been spending a lot of time in the Underground. And I've found some pretty rare and useful stuff, for anyone who's interested.
Pokemon
Omanyte x2
Anorith x2
Cranidos x1
Aerodactyl x1
Stones
Thunderstone x7
Sun Stone x3
Fire Stone x2
Water Stone x1
Leaf Stone x1
Shards
Blue Shard x3
Yellow Shard x2
Green Shard x2
Red Shard x1
Plates
Splash Plate x1
Toxic Plate x1
Earth Plate x1
Iron Plate x1
Mind Plate x1
Fist Plate x1
Miscellaneous
Heart Scale x4
Star Piece x1
Rare Bone x1
More common Items like Everstones, Ice Rocks, Hard Stones, etc. weren't included in the list because they aren't really rare...
Anyway, I'm gonna keep digging; if anyone takes any interest in these Items and/or Pokemon, please let me know. Maybe I can start my own custom Underground Swap Shop...!
~CuleX