When you listed the Landrover steps, you said that you had a quantity of two available. To eBay, that meant that you had two pair, or two sets of side steps. You accepted an offer for them yesterday, which sold 1 pair or set, leaving the listing still active with a quantity of one available. So someone purchased the last pair today.
If you only had one pair of side steps available, then you should have listed that as a quantity of one, not two.
