If you are looking to purchase keyboards second hand on geekhack, then you will need to use classifieds.
If you are looking for aftermarket keyboards, it depends but for most oems, you can find good deals for stuff like model ms on ebay (especially if you are in the us), if you want aftermarket topre than places like mercari are great for finding cheaper prices for stuff like hhkb, you will need a proxy but buyee which is what I used is integrated into mercari now, and my hhkb came in fine.