It's kinda difficult to say that something is only worth $10 when you've just sold it for $500. For that to happen, a judge would be able to suppose that taking your $500 and giving you $10 in return wouldn't make you unhappy.

Sure, the way that most countries' customs work when it comes to value declarations in shipped items (trust) mean that they aren't likely to catch on to it, but the fact remains - if you happily sold it for $X, then that is it's true value to you. Now the buyer may consider if *more* valuable than the price he paid, but I don't think that's the issue here.