My empeg belongs to me, unless I sell it. If someone
has it in their posession, it still belongs to me.

Correct

If someone steals my empeg, wipes the hard drives and sells it on ebay, it still belongs to me

Wrong. If whoever bought that Empeg from the thief in good faith (and I don't see why this wouldn't be so when you buy something via Ebay), then the Empeg IS HIS property and WILL REMAIN his property, EVEN if the thief gets caught afterwards.

Best case, when the thief gets caught, you can demand that he repays you for the damages done (the value of the empeg), but you WON'T get the empeg back because at that point it has become the property of somebody else.

This is the law in Belgium anyway. Strange but true. Believe me, I know. I deal with these matters on a daily basis.