Stat wise: Get enough Dex so that you can effectively hit the monsters that you're killing for Exp. After that, I'd say go with Strength and Vitality (Or Agility if you have some Flee gears!). You don't really need Luck at lower levels.
Skill wise: I'd say go for maxed out Pushcart first. After that, Discount and Overcharge. Enlarge Weight Limit also comes in extremely handy as well(And you need it at level 5 at least for Pushcart anyway). I also recommend getting the two Merchant-class exclusive skills Crazy Uproar and Cart Revolution from the special skills NPC located in either Prontera or Alberta.
Training spots: Poison Spores in Geffen Dungeon (Go to Geffen city from the warp girl, and just head north into the tower) is quite nice at lower levels.
