I have a macbook pro that I got in 2012, it is 2.8Ghz , the intel core is i7 and the memory is 4 gb. How much would it be worth approx if I sold it

I use Gazelle's website to get a good idea of the value of old electronics. You can probably get a little more if selling directly to an individual. At least this gets you in the ballpark.

Sell Your Cell Phone and Electronics Trade In | Gazelle (http://www.gazelle.com/)

Good luck,
WallyH

Going for about $800 tops on eBay right now.
Expect less on craigslist, maybe $600 tops.
You didn't say screen width and some other info...