Can you upgrade my MacBook Pro 2013?

No, you can't.

You should have done that before buying your macbook pro using Apple’s customisation of hardware options. Unfortunately, its too late because the processor is soldered to the logic board itself.

But you don’t have to regret much as the difference between i5 and i7 processors is negligible in day to day task.

- Granel

YYou will get the best price selling privately. You can try Craig's List and meet potential buyers at a public library. Posting notices in college dorms and student unions are another good bet. Selling thru Amazon is a third option. Gazelle.com ttends to pay the highest for a straight sale but you won't be happy. The saving grace is speed and convenience.