I'd prefer to buy a brand new system and install it myself but for the technologically challenged that don't want to take the time to learn how to do it they should pay for its condition assuming its not new + 3/4 hours spent installing it x minimum wage.
That would be the most fairest way to accurately price something like this, at the same time the market itself should dictate what something is worth. With that said take it to eBay, price it at its MSRP and describe in detail what arm9loader is and its benefits. Let bidders decide its worth.