As far as I know, it's not against the law to use multiple advertising channels (eBay policy is debatable - they once had a specific policy prohibiting it, but it's disappeared from the site), but you could wind up in a sticky situation if two people purchased and paid for the item, that then - legally - creates two contracts, so you'd have to break at least one of them.

I would personally only use one site where an item could be purchased on-site - if there's no "buy / pay now" type of facility via the site, I can't see the harm in using it to advertise, it'd basically be a classified, and you'd be in control of whether it sells (and can end the eBay listing if it does).