You are correct. Maybe.

The buyer is offering a round figure, ebay are taking the BPF from that, and you are now technically paying the BPF.

Try replying with a message such as "Thank you for your offer of £258.92, if you could make it a round £270, then we have a deal. I have sent a counter-offer of exactly £270.", and with the message, send a counter-offer of £270. This will cause a buyer malfunction, as they know what they offered was not £258.92, they know you countered with exactly the amount they originally offered, they know the figure presented to them is different.

They may walk away in a fit of pique, or they may contact ebay to ask what's going on.

Have fun!