mmh lets see
if you have no clue about the item price i would say ask in the pricechek megathread and offer lowest price you get answered with, and if you are not shure about the price just lowball and shit.
good items to sell are mostly forces,laxes,textures,hairs and 3d items but for the last 2 you have to chek people threads and how fast they get sold in market...ect
i'd also suggest staying away of misc except if you get it insanly expencive coz they are damn hard to sell.

now for how to start:just open a big buying thread and sell items on market for high prices and never buying an item that is closely priced to market