For all intents and purposes, when the insurance company declares your car a total loss, they are just offering to buy it from you at what they believe to have been the market price of your car had it not been in a crash (you can haggle with them).

A few years ago we had a nice car that was in a moderate crash. The damage estimate was around $20k, but the insurance company said that they would rather have the car for the parts than fix it - it had a bunch of options that were in very short supply at that time. We worked out a deal, and they cut a check. I think I paid $80k, drove it for a year, and sold it to the insurance company for $75k. LOL.

"All that is required for a vehicle to be a write-off is that it would cost more to return to marketable condition than the market value it would then have."

Yes seems to agree with what I said. "marketable condition" i.e. not undriveable.