buy a 25 pound bag of rock salt its cheap. mix in washtub with the hotest water you can get. dont add a lot of water, you need to send a salt rich dose of water down the line, then go out for the day, let the salt laden water sit in the line..... till late that night.

this will kill any tree roots but not hurt the tree..

If I were the OP I would get the line snaked and camera inspected, the OP can likely go after the old owner for the costs of repairs, if the problem wasnt disclosed before sale