Kind of misinformation.
The tanto sword there looks to be in good condition.
Without looking at a signature, it's value is unknown.
The tanto was used primarily for utility and personal defense when invited indoors. Seppuku in itself was rare and a tanto was not always necessary but was common (a wakizashi could also be used, or a ritual blade, just about anything sharp). The cut would be across the belly, and it was considered to be courageous and more "worthy" of cleansing your honor to also cut vertically not just horizontally. It's supposed to be a lot more painful to cut vertically.
Secondly an assistant which would cut off your head was not always granted, it was up to the samurai's lord. Seppuku was also committed (and more famously) to attempt to show the Lord than his current course of action would be bad for the lord.
In Japanese culture, it was said those whom were honorable would have clean intestines while those from are dishonorable would have dirty intestines. By committing seppuku, one's intestines would be shown, and if they were clean they could sway the lord from their course of action.
Japan has a culture in which respect for positions, and ettique is incredibly important since it was incredibly disrespectful to the host, and also a big practical problem since a sword was large and homes tended to have tight hallways where a 40" sword would be unwieldy. Disrespect could be grounds for dueling to death, or death it self (incase you were a commoner disrespecting a samurai).