Unfortunately, you cannot sell your MBP with an OS downloaded from the app store as the license is for the Apple ID used to obtain it (you) and it is not transferable. Any buyer would not appreciate needing your Apple ID and password for updates or a reinstall. So, you need to erase your machine and reinstall the original OS on it. Using internet recovery won't work on yours as it came with an install DVD. If you no longer have that, you can either order one from Apple or get a retail Snow Leopard disk to install.
http://images.apple.com/legal/sla/docs/OSX1011.pdf
Excerpt:
3. Transfer.
A. If you obtained the Apple Software preinstalled on Apple-branded hardware, you may make a
one-time permanent transfer of all of your license rights to the Apple Software (in its original form
as provided by Apple) to another party, provided that: ℹ the Apple Software is transferred together
with your Apple-branded hardware; (ii) the transfer must include all of the Apple Software,
including all its component parts, printed materials and this License; (iii) you do not retain any
copies of the Apple Software, full or partial, including copies stored on a computer or other storage
device; and (iv) the party receiving the Apple Software accepts the terms and conditions of this
License. For purposes of this License, if Apple provides an update (e.g., version 10.11 to 10.11.1) to
the Apple Software, the update is considered part of the Apple Software and may not be
transferred separately from the pre-update version of the Apple Software.
B. If you obtained your license to the Apple Software from the Mac App Store, it is not transferable.
If you sell your Apple-branded hardware to a third party, you must remove the Apple Software from
the Apple-branded hardware before doing so, and you may restore your system to the version of
the Apple operating system software that originally came with your Apple hardware (the “Original
Apple OS”) and permanently transfer the Original Apple OS together with your Apple hardware,
provided that: ℹ the transfer must include all of the Original Apple OS, including all its component
parts, printed materials and its license; (ii) you do not retain any copies of the Original Apple OS,
full or partial, including copies stored on a computer or other storage device; and (iii) the party
receiving the Original Apple OS reads and agrees to accept the terms and conditions of the
Original Apple OS license