I currently run the seforim library in my yeshiva. this consists of purchasing seforim etc. Is there a din that there has to be two gabbaim overseeing the funds (as of now I recieve cash from the yeshiva pushka and other donations).
Also, do I have a resposibility to find the cheapest sefer, so as not to make the yeshiva lose money, or can I buy locally for my convenience, although it is more expensive?
It is permitted to function as a single individual in running the seforim library, and similar public functions. Although charity collectors have to collect in pairs, this is only because of the issue of serara (authority over the community), whereas in other public dealings a single person is trusted. However, you should avoid any action that might cause you to be under suspicion, and it is worthy practice to keep a careful tab on moneys received and spent, to avoid all suspicion.
Conerning buying books at the cheapest price, it stands to reason that your appointment does not mean to take you away from your studies in yeshiva to a greater extent than necessary, and it would be permitted to purchase seforim at the local book store. It may be worth trying to reach an arrangement with the bookseller to sell for a reduced price to the yeshiva.
Sources: See Yoreh De’ah 256:1; 257:1-2; see also Taz, 257:1. The advice of giving an accounting is given by Rema, 257:2.