Sorry that, wasn’t very clear. I’ll explain with an example:
I make a beacon using my last five plots, and I now have no more left. I want to sell this piece of prime real-estate, and to do so I put a pay lock on it, and set the price I want. You then come along, inspect the pay lock and decide that you want to buy the beacon for that price. If you were to have less than five available beacon plots you wouldn’t be able to buy it, but luckily you have eight available, so you pay the money, and the beacon is now yours with everything in it under your control. You now have three beacon plots left, and I have five left.
I hope that explains it better.