I frequently advertise items on a SA on-line auction site (similar to eBay) and on Gumtree. If someone buys the item off the Gumtree ad, I delete the entry on the on-line auction.
Obviously you have to be careful that there have been no bids on the on-line auction, or you cannot delete the auction, unless you have multiple items.
I think policing whether an ad also appears on eBay would be difficult, you will just have to rely on the posters' integrity