The buyer should decide beforehand if the shipping should have insurance or not, and how much the package should be declared for = how much it is insured for.
The buyer might want to declare the value to be lower than what was paid only if there could be import tax. If there could be import tax, then the buyer should also decide what the description text should be on the customs form.
If there is insurance and the package is deemed lost, then the seller has the responsibility to request the insurance money and pay the buyer.
If there is no insurance and the package is lost, then nobody gets anything.
These have always been the implicit rules for when I have bought and sold stuff on eBay and message boards and we have not said anything else. If you want another arrangement, then you should agree on it beforehand.