The only keyboards that cost $75 that I know of are the Northgate OmniKey series (long out of production) that work on the IBM PC, Amiga's , and other systems. You can remap the keys and rearrange them. I have one on my desk that I use on my PS2 KVM (nice old IBM type feel).
I dont think the $100 500 will sell, but you never know.
Alot of ebay auctions are bid up by fake user accounts, you see stuff sell at crazy prices only to be relisted days later. This way people who dont know better use older auction completed prices and bid accordingly on new auctions making some lucky seller happy. I say know what your buying including model number information, options, conditions, auction fine print before you think about bidding on something. then and only then do you decide the maximum you will pay for the item. Some type of stuff is put up for auction all the time, if you dont get a good deal today check back tomorrow. People who get caught up in the "got to have it now its rare and nobody is going to outbid me" are the ones paying outrageous amounts of money, or just have so much money that it doesnt matter what they pay for an item. A true collector will keep looking and find what he wants in good condition and at a good price.
To sum it up if you dont like the price dont bid on it. When people dont buy items at inflated prices they WILL come down. Alot of the prices paid for actual "rare" computer equipment has dropped quite a bit since the stock market and US economy took a dump in the last year or 2. People who dont have jobs are/will be selling thier equipment to pay the bills and when you see multiple rare items at auction on the same day the price will drop. I am hoping to pick up a A1200 by this method (not that its rare just overpriced).