The shopkeepers pay ridiculously low if you don't have any perks in Speech (like I don't). They pay 100 golds for an item they then sell for 1200 golds. Zenithar's +10% doesn't seem to make that much difference. Yet I wouldn't really want to use perks for anything else but combat (including sneak) and smithing related. Monsters and other villains are just so tough in Skyrim.