How do I woke how much my iMac is worth

In addition to the above suggestions that will give you an idea of probably what the max amount you might expect to get you can also try getting offers from the used computer resellers like Gazelle. A simple Google search will turn up several of them. I sold an i-Mac to Gazelle a couple of years ago and got what I thought was a reasonable offer for a 9 year old i-Mac and took the offer. This market will typically offer you less than a peer-to-peer sale on Ebay, etc. If you are looking to buy a new Mac you may also want to consider a trade-in at Apple. See this link for info.

+1 You can also look in Craigs List or MarketPlace for what similar Macs are asking in your area. Try to get an idea of how long they have been advertised. If they have been on the market over 2-3 weeks they are probably overpriced.

Selling anything used is an odd sort of business. If the potential buyer desperately wants what you have, the price can be high. If there is no demand for what you have the price will be low.

Look for similar units on resellers sites to get an idea. If you trade it in for a new Apple, Apple will make a trade in offer. But that is subject to what the company making the offer has to say about what its worth is. Generally, trade in will be less than a private sale.