Stealing an item is pretty simple, many items inside houses and inns are considered "owned" so just keep an eye on Equipment with a Red title that has a % chance to steal.
Once you've successfully stolen an item, the only way to sell it is to head to one of the 3 travelers camps throughout the lands and sell to a merchant there. The closest camp to the start of the game is Star Camp. The Camp in situated in the eastern part of Dalentarth in an ancient forest. It is found South-East of Didenhil, North of St. Eadric's Mission and South-West of Warden's Bridge. The camp has two entrances: the northern one from the region of Lorca-Rane and the Southern one from Haxhi.
Head here and sell any items that are stolen (or if you've placed them in your Junk folder you can just choose the "Sell All Junk" option and vwalaa, trophy attained.