I was involved in a similar situation once, landowner versus company (dropped road trailers). After a lengthy (and likely hostile) back and forth, the company finally sent the landowner a certified letter to begin charging Demurrage for the trailers in question, @ $50 per trailer, per day if they weren't released for pickup. The landowner relented and let the trailers get picked up, I think he either realized he would lose that case, or just wanted to be done with it.
If the landowner wants to pursue a lien on the buses, he needs to get on with it, otherwise he's interfering with (presumed) legal ownership and retrieval of property. Buyer needs to go to local court, get a court order to retrieve the buses, and get cops to go with him to do it (the landowner can't really stop it at that point.)