A general 'rue of thumb' for used audio equipment which is 'just like new' is 50% of list price.
For 'some' equipment which are 'classic' (if you also have the box) is 60% of new list.
Very very few items can be sold for more than 60%. Some examples I know of would be Audio Research Sp-10
Infinity Reference Series V speakers, Vendetta phono boxes, Garrard pro idler drive turntables, Marantz Model 7
A small and highly prized list.
For the average bit of gear, particularly if old and no box, with a few small flaws.. 40% or 30% of list.
Then plenty of audio crap which is worth next to nothing.
Mac gear would fall in the 60% range if perfect and you have the box.
As proud cheap bastard who buys most of his equipment secondhand or on sale I can agree with most of this. I will offer some other thoughts:
1. If the product is current or can be updated to current AND the current product is in high demand, your price will likely be higher. This is doubly so if there is a long waiting list for the new product and/or the price has risen significantly from the original purchase price.
2. Resale tends to be higher if the product was a "giant killer" or is relatively inexpensive. There tends to be a price floor that supports resale pricing for high value products.
3. Resale will tend to be higher if the product has a near-universal appeal. Esoteric, large, heavy and really expensive items tend to fetch a lower resale than those that more easily fit into a second owner's system (or mailbox).
4. I've found that surveying the current market is appropriate for most sales. I like to check what is currently being sold on Hifishark, Ebay actual sales, and a smattering of other sites including Audioasylumtrader, Audiocircle, etc. This will give you an idea of the closing prices and also what you are competing against.
5. If a product is not in high demand, you may need to be patient to find the right buyer. If you need to sell quickly, you'll need to be lucky or willing to accept a lower price.
6. Don't overlook local sales (e.g., Craigslist). I am in the NYC metro area and there is not a lot of competition for high end audio equipment on CL. This is especially good for items that are impractical to ship or for buyers who wish to audition first. Beware of the "We buy tube equipment" guys, though. They will generally only give you about half the expected resale value.