If they're really at 5V - and have been there for any length of time - they probably are unrecoverable. If you can, they'll likely have little capacity. A 6V lead-acid is considered "dead" or 100% discharged at 5.25V! It's very hard on a lead-acid to be discharged that low, and they sulphate rapidly if left there.
However, if you only paid $30 for the set, you'll still come out ahead should you decide to purchase new batteries to play with. There's a core charge applied to any new battery you purchase, you get that back if you turn in an equivalent-size battery. I just bought a replacement battery bank and L16-style cores are $30, T105-style cores are $20. I upsized my bank from T105 to L16, so my total core charge was $240, and I was credited $160. (8-battery 48V bank.)
I don't know if anyone will pay you that just for the batteries, you might have to take them directly to a scrapper if you want to get the cash without buying new batteries.
I got six L16s the same way you did - a place I was working at had pulled them from a floor polisher. I got them for free, this machine had sat unused - and off the charger - for several years. They were worth exactly what I paid. I did have a few hours of fun tinkering with them. They actually took some charge, but would collapse at the first hint of load and you could watch the voltage steadily drop on the voltmeter as soon as they were taken off the charger.