I was at a gun show in Tulsa years ago and one man at a table was buying all the Japanese swords he could find. He purchased one that was a real mess. Whoever had brought hit back did not know the value of it so he used it as a cane knife, sharpened it on a grinder, and when the handle came lose he wrapped it in friction tape.
The table dealer bought it because it still had a good hand stamped signature under the handle.
I bought a nice short sword a few years ago. I paid $100 for it. The furniture looked old and utilitarian, the blade was heavy and did not rattle around. It had a signature on the tang. For $100 I took a risk. I finally was able to authenticate it from the signature, it was made in the late 1680s to early 1700s in Edo by a well know and very prolific sword maker. The actual value was closer to $1100. Great deal right, well actual the rust on the blade meant it would have cost me around $1000 to have it polished. The seller knew exactly what it was worth and was being honest when he sold it to me. It hangs on my wall, I may get it polished to hand down to my son.