I second the vote for ClickyKeyboars as a supplier, as I am batting about 60% favorable off of eBay, 40% not so good. I have recieved two ebay boards that were disappointments - a worn out white M13 (lousy key action, maybe something stored on top of the keys?) and a 401 that was described as fine - and wasn't, with missing keycaps and a broken keystem that were not mentioned - and a "period" key that requires three times the force of all the other keys on the board. Luckily it was cheap, and will provide parts for the others, as needed.
If you deal with ClickyKeyboards, you will be sure it isn't junk, and have recourse if you do have a problem. Because of shipping costs, it matters to get a good one, as returns cost you, too. I recently got a brand new one from them, and it was fun being the first one to unseal the box!
My personal feeling is the boards made by IBM, 1990 and prior have a superior feel - but I have a very small sample of about six Model Ms right now. Your mileage may indeed vary. Cheers, TPF