As a general rule, I almost never offer the seller's asking price unless:
1- The asking price is already laughably low
2- Its a hard to find item on the used market (while still being priced reasonably)
In my mind a used product is typically worth about half of its new price (in excellent/good condition). You get no warranty in most cases and no means to return the product if you're not satisfied. All that said, $150 is probably 'reasonable' unless you're talking about the cheaper "silver" performer. Then I'd offer closer to $100. If the item has been listed for several days (or weeks), I'd offer $110-120 and see if they bite. You can always go up in a negotiation, but rarely can you go back down.