Whether you buy used, refurbished, demo or new*, insist on getting the original discs that came with the Mac. The reasons are the following:
1. The seller can't hold on to the original discs legally, as they are licensed only for use with the machine.
2. The discs contain valuable hardware diagnostic and directory repair utilities.
3. The discs contain everything that does not come with the retail operating system of the same version, as Apple bundles certain software packages separately from the main operating system.
4. In some cases, such as Intel Macs, the version of 10.4 that comes on those discs is the only one which will boot the machine unless you clone backup your data to an external hard disc or have the same Intel Mac model to load into target disk mode with.
5. Additionally these discs have system specific information as these articles explain:
Mac OS: Versions, builds included with PowerPC Macs (since 1998) - Apple Support
If the seller lost the discs, ask them to call AppleCare:
And send you the discs. If they won't give you the discs, request a refund and return the machine to them.
If you can't contact the seller anymore, call AppleCare at the same link above and see if they will send it to you. There is usually a fee for them.
Similarly, you should always give up the original discs (and not copy them for yourself) when selling a Mac.
Note starting with July 20, 2011 Macs, no Macs came with pre-bundled Macs. Since the MacBook Pro lineup wasn't updated till October 2011, that particular lineup was one of the latter ones to no longer ship with pre bundled discs. Apple included a special restore installer on the machine for 10.7 or later.
* When buying new, the discs are usually packaged with the manuals in the box the computer comes in and inside an envelope. If for some reason they are not, call AppleCare at the number above.