That means "소장하다, 소유하다, 갖고 있다, 보유하다, etc."
"갖고 있다" is an easy phrase of it.
"If you buy books instead of borrowing them from the library you can (keep) them for as long as you want."
"책을 도서관에서 빌리는 대신에 사시면 얼마든지 (갖고 계실) 수 있습니다."
빌리다: to borrow (빌리는: a form of the verb that modifies a noun)
대신에: instead of (대신 is a noun, that's why 빌리는 was used.)
사다: to buy (사시면: the honorific form of "사면 (if you buy)," and -면 is similar to "if.")
얼마든지: as long as you want
갖다 (=가지다): to have, possess, get, etc.
-고 있다: a phrase similar to "-ing" (-고 있을: a form of it that modifies a noun, and means possibility or future tense. -을 does those roles.)
-ㄹ 수 있다: be able to (갖고 있을 수 있다: be able to have/possess (litterally: be able to be having))
-ㅂ니다: a kind of honorific ends
Hope it helps. :)