The price mainly depends on the following aspects. All are equally important and must be considered while fixing the price. Unfortunately we don't have a formula in place, but just keep these in mind and arrive at a realistic estimate. Now we understand that you might want to recover as much as possible, but that might not be appealing as an offer.

1) How long ago was the product purchased?
If it was purchased recently, then by all means fix a price without deducting too much from what you’ve paid for it.

2) Warranty?
A product still covered in warranty, with at least half the warranty period remaining can justify a higher price. On the other hand, if a product is out of warranty, a seller has to keep in mind that the product can go bust and might have to pay for repairs if necessary.

3) What Condition is it in?
If you’ve used the gear extensively, it'll show. So price it accordingly. People like pretty things. If necessary, spend a little to make them look as close to new and maybe you won’t have to compromise too much.

4) What's the current selling price for a new piece?
Always consider at what price a buyer could get a new piece. The difference can't be too little otherwise there is absolutely no way a buyer will be inclined to buy pre-owned gear.