If he used it for anything illegal and its tracked thru to you being any way in the supply of the sim before or after registering the law looks at you as facilitating the crime. Think 3 times
I'm no law expert. But there are dozens of sale adverts on ebay etc oz, uk...I cant see that they are all risking being implicated in a crime. E.g. http://m.ebay.com.au/itm/131538408902?nav=SEARCH
What if I sold a mobile phone that went on to be used in a crime???
@op - I'll err on the side of caution and direct you to the multitude of ebay sales.
And that's a postpaid service, I'm 99.9% sure the OP is after a registered number not just a sim card from Australia