All About Pocket Knives

Usually, I make a point to NOT buy used sharpening stones. I put a pretty low bid on these synthetic Norton stones, as the seller had a couple of other items I was also meaning to purchase but lost the bids on both, and forgot about them. These four stones ended up costing me almost $20.

They look loaded with metal, not much different than the same Norton of this type I've had since I was a teenager. One Norton stone is new, but they aren't expensive. I have one in my toolbox, and it cost like $5. I figure that the sealed one is still perfectly usable and that if I clean the others good enough, they'll be handy to have for quick sharpening jobs as needed.

Are these salvageable, or am I out $20?

Those small stones are salvageable. If you want to clean them I would suggest putting them in very hot soapy water a time or two and maybe scrub them with a green kitchen scrubber. Although they look usable as they are.
Because they are small stones they haven't been used very hard.
The country is awash in sharpening stones. But not everywhere.
If you can buy larger used stones at flea markets or yard sales I would highly recommend it. Be choosey about used stones. You probably don't want them too deeply dished (as some of mine are). They will sharpen a knife.
For $20 you can upgrade your stones by a long way with one good sized stone. Shoot for 8 inches. You'll be glad you did. OK, go for 6 inches. You'll be glad you did. Larger stones are so much easier to use. You probably know this.
Use water.

On old used stones that are really grimy with caked on sludge and steel/grit slurry embedded in the pores, I give them a blast with some spray can brake cleaner before soaking them in the mentioned hot soapy water. It will take off some of the really heavy stuff first and speed things up. Do this by laying the stone out in the middle of the street and blasting it, one side and then the other. Take the usual precautions when using brake cleaner.