I depends very much.
Since you already used it it cant be mint condition I think that it is for used condition.
It is not worth very much but if I were you I But if I were you, I would save both of them and keep them in great condidtion, However Here is what I think they are worth.
Did you have the Yellow pikachu Game boy color?
Limited edition Pikachu Gameboy color 70-90 $
Blue Version 10-30 $
Yellow version 10-30 $
Both of this games in mint condition would be worth 100-200 $ so if you had bought 2 more Pokemon Yellow/Blue when you bought them many years ago. You could sell them now and earn much more money than what you bought them for just to know :D.
Hope this helps