Look for wands, staves and scepters with spell bonuses on them. (Sorceress orbs, necromancer heads, barbarian helms and druid pelts typically don't sell for much.) The higher the spell level, the more money they sell for. They typically sell for up to the maximum gold an NPC will give you. Another good source is superior (non-eth) armor, especially exceptional and elite stuff.
Failing those things, the full stacks of the various poison and fire based throwing potions can sell for a lot too.
Failing those things, the full stacks of the various poison and fire based throwing potions can sell for a lot too.