I agree with Suren. I think the two verb forms differ in terms of time aspect, that's all. булсан shows an action that happened in the past-buried, while булаастай indicates that something is still there- still buried. I'm not a professional Mongolian teacher but as a native speaker and English language teacher I tried to explain using my knowledge in linguistics.

Булах - булсан - булаастай. Literal meaning is "булсан хэвээрээ байгаа". I guess both булсан and булаастай would translate into English as "buried". На русском, булсан - закопал, булаастай - закопанный.