The cheaty-est store item I know of is the produce stand. Join the gardener career for an extra discount, then buy out the books and then sell them for a HUGE profit.
My favourite non-store ways to make money are gems and transfiguration. If you don't want to use the collection skill challenge slot, buy a miner from the consignment store and mine until you get tiberium, or get a dog. Then cut it at the consignment store. Wait til all the bits grow, and then consign them.
If transfiguration is a go, then buy out all the gem dust at the elixir consignment store and any gems from the consignment store. Turn them into soulpeace gems and cut them and sell them (consigned of course for the bonus).