That's an interesting question that I have pondered on several times having cleaned out multiple old stock ponds. I've always opted not to use the stuff. On one hand, it seems like it would be ok and maybe even good, but on the other the risk of presence of heavy metals and/or chemicals has always swayed me away from using it.
If it was from a pond I had made/owned all of its existence, I might view it differently but who knows what someone may have put in there 50 to 100 years ago. Carcinogenic chemicals especially herbicides were very commonly used in old ponds to kill out weeds and that stuff can be long lasting residuals. Copper was a common treatment in the old days.
I would recommend a soil test on the stuff before purchasing.
It is not easy stuff to work with. Takes a long time to dry it out and requires spreading to get it dry in my experience. Ponds generally have clay bottoms to prevent leaking and it is likely the stuff, pond muck, will have a good bit of clay in it which I would not want for garden. Whoever is selling it gets a star for creativity in my book as most folks have trouble getting rid of the stuff.
A bucket load can be most anything, but I'd guess they are talking about a frontend loader bucket on tractor which usually is something like 3 cu. yards. If the stuff is wet, be careful because it will be extremely heavy and can be very messy/smelly to work with.
