Hypothetically speaking, the book in question would determine whether there is or isn't a "bag to be left holding". For instance Lets say the book in question was a TTA 13. Bought as a 3.5 or so before anybody had an inkling that the GOTG franchise would emerge and tat the book's status would change from "just another TTA/Atlas pre=marvel horror common" into a sought after super key. The book's market value explodes exponentially, and a restored 3.5 soon becomes 2 to 3 times the value of what an unrestored 3.5 fetched when you bought it as unrestored. This is now a book that restored or not, you are not going to want to return it.
There are loads of books like this. TOS 50, MSH 13, MSH 18, etc., etc., books of nominal value and interest and then suddenly, PAM!@ They hit. So, the question of whom is left holding the bag on a book where resto is discovered at a later date than when purchased can only be properly addressed once a title and number join the elements of the question as important details.
Also to consider, the proportion of market value, the difference between market price of books in the lower grade ranges, between unrestored and restored is slim to begin with, in some cases realizing prices that are very similar!