Given that sellers shouldn't receive messages saying "If your offer is accepted you'll need to make payment", I'd say that the most likely option is that this is some kind of eBay glitch, especially as this kind of message is aimed specifically at the buyer. As the seller making a counteroffer in response to a buyer sending an offer that is too low to be deemed acceptable you shouldn't even be seeing this message.
In the event that you are subsequently sent an eBay message telling you to make payment and it relates to your own item that you're trying to sell, contact eBay Customer Services about the matter and draw their attention to the erroneous message, stating that you are the seller and not the buyer. If you can get through to somebody with an ounce of sense they should void the message and not hit you with an Unpaid Item Strike. It would be a bit stupid of eBay to say the least if they gave you an Unpaid Item Strike for not making payment for an item that you were actually trying to sell on their site!
Should you end up having to contact eBay Customer Services about this matter contact them using the following link: https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid. Once you have clicked on the link click on the blue Have Us Call You button at the bottom of the screen. The lines are open between 8:00am and 10:00pm from Monday to Friday and between 9:00am and 6:00pm on Saturday and Sunday. However, it's best to try first thing in the morning, as that way you stand a far better chance of getting a call from somebody in Ireland, as opposed to an eBay representative from eBay's Philippines-based call centre, which could potentially cause problems with making yourself understood if the fluency in English of the person you are speaking to leaves a lot to be desired.
