I would take pictures of the area now. I would also get pictures from before showing the pavers in place. If there are pictures from the real estate listing showing the pavers, include those as well.
You have a lot of options, however, the separation of legal terms, facts, statements, and the sense of robbing your landscape, all take on different directions.
A) you could file a criminal complaint for stolen pavers.
B) you could file a civil claim for the removal of pavers.
C) you could estimate the cost for the project to be completed from bottom up, with pavers, with cement, with gravel, and present this information to the seller for resolution of the removal of pavers.
D) do you know for certain the seller is the one who took the pavers?