My understanding as a jailhouse lawyer in a completely different jurisdiction (Disclaimer: those who rely on jailhouse lawyers often end up as their cellmates) is that the legalities would be summed up by the negligence laws in the jurisdiction in question. Depending on where this happens, you probably have a duty of care to not cause damage that is reasonably foreseeable. If you break that duty, you can be probably held liable for damages.
However, even if you do decide that juggling those Faberge eggs would really be hilarious and then drop and shatter them, that doesn’t necessarily mean you are liable for the whole retail amount. The store can also be held negligent, due to the doctrine known as comparative negligence. In that case, the court would probably divide liability between the customer and the store according to their respective degrees of fault. Even if the store isn’t negligent, the court would probably only award the wholesale price plus any restocking fees, not the retail price or fair market value.
Personally and ethically, I’m not paying a fucking dime until ordered to by a court.
Ethically I would feel obligated to make the seller partly whole. It’s not their job to babysit my theoretical kid, after all - but it is their job to babysit their stock. If my kid gets their hands on their stock and breaks it, then I’d want to know the seller wasn’t doing something careless or stupid that allowed it.
And this exchange: “So how much was it?” “$4000” is immediately followed with a “Yeah, right.” And also with me immediately concluding that the seller was pretty much wholly responsible for the accident, since only an idiot (or a scammer hoping for this very scenario and/or insurance fraud) would leave a $4000 item out unattended in a flea market.