A new iPhone can be expensive. If you want to offset the cost by selling your old one, now might be the best time to do it.
Resale values start dropping by about 10% in September and October, according to eBay (EBAY) data analyzed by NextWorth, a gadget trade-in site. That's generally when Apple (AAPL, Tech30) releases new versions of the iPhone.
Also read: Ross: NAFTA is job one