Thrice, I would hazard a guess that you have this lens in T-mount (I seem to remember that's how it was produced, but I'm too lazy to go looking for the info). Now you have to go back to eBay and look for a T-mount to K-mount adapter. It'll probably cost you more than the lens did.
#1 Rule of eBay: Research your target product before bidding!
Of course, $20 for this lens is
very good. I wonder how I didn't see it?
EDIT: Oops, I just read this, and I meant to write "TX" mount (similar to the "T4", its predecessor). This was Vivitar's "adaptall" mount of the 70's.