There are a fair amount of places where used equipment is sold where you can look to recent sales for comparison, for example you can search past postings on the Buy Sell Forum.
Another convenient way to look at recent sales of a particular item is to filter by Sold Listings on eBay.
- Search for the name of the machine you are selling or looking to buy.
- On the filters on the left, Select Sold listings.
- To get the most accurate reflection of what the current market is doing you will want to keep your search sorted to End Date: recent first.
- Looking through these sales you can get an idea for the average going rate of that time.
Note that there may be a wide range and even some outliers. Listings where the machine is in extremely good condition or comes bundled with a bunch of accessories will probably have sold for higher. Machines in dubious working condition or known to need a lot of love will (should) have been priced lower. Hopefully there are enough reference listings to be able to get a good feel for those ends of the range if necessary as well.
- You can also sort the sold listings by "Condition", clicking "Used" can help you weed out items priced as "New" or "For Parts".