Was the sink used with some detrimental substances that you are worried about? If not, then anything "bad" from the concrete itself is long gone and you'll have no worries.

Sealing hypertufa is negating one of the good things about hypertufa: its ability to breath. Another reason why sealing would not be called for unless you have a warranted concern.

That said, there are sealers made specifically for concrete. Find them in the paint department of places like Home Depot.