It's difficult to pin down a value for used gear.
Sometimes half of retail is what some things end up selling for.
Older HID canister lights, BCs, Shearwater Predator and Liquivision X-1 computers, and fins come to mind. These items frequently sell for 50% or less of the retail price.
Depending on age and condition, you can get more, perhaps up to 75% of retail.
Newer Shearwater Petrel and Perdix computers, lithium battery equipped primary lights, some newer brands of BCS, or Halcyon backplate/wing setups are some examples. Keep in mind doing that 75% is pushing it.
Personally, I'm not going to pay much more than 75% of the retail for anything that's used.
I'll look for a new one at a discount instead.
How motivated are you to sell?
If you want to move something quick, and want to free up some cash, target around 50% of the retail price.
If you aren't in that big of a hurry for the money, and are selling just to get rid of some excess gear, start around 70% or so and go from there.
It really depends on age, condition, brand, and how quick you want to move it.