I have never tried to make money on keyboards, or break even, for that matter.
I have sold quite a few keyboards on ebay, for various reasons, and, yes, there have been times that I have bought keyboards at yard sales or thrift stores for $2 and resold them for $20-40 (after thorough cleaning and testing), but those meager earnings have not even remotely covered my expenditures overall.
The fact that I have had a really nice, well-balanced, lightly-pingy bolt-modded Model M up for well over a month and it won't sell even at $60 proves that there is not really a lot of easy money out there.
My thought is that when I find something that I really like, I want a really top-shelf specimen of it, plus maybe a 2nd for backup, and MAYBE, at most, a 3rd if my son wants one (my teenage daughter wants whatever looks shiny and tiny, and my wife thinks that the best keyboard in the world is whatever Apple manufactured in the past 6 months).
Model Ms are an exception, and I think I need a handful to cover the various changes and possibilities through the years.
But, bottom line, when searching for near-perfection, it often takes buying 3 to get "the one" that really rings your bell.
I would not sell anything on ebay that was not, at the minimum, working properly (except that I once sold a Northgate Omnikey with a bad key, but advertised it as such), and pricing it according to its condition. Alas, I may be the exception, but I try to be fair.
So yes, I have "churned" or "flipped" about 2 dozen keyboards on ebay, but I feel that it was all on the up-and-up, and it has left me with a half-dozen really prime pieces, and a several-hundred-dollar hole in my pocket.