It's exploiting the save system. It's essentially item duplication (works with everything but guns) it's just called the egg glitch becuase rotten eggs are the most valuable item you can do it with. It has to be done in co-op.
1. Acquire a rotten egg.
2. Remove all items but the rotten egg from Both Chris and Sheva's inventories.
3. Start chapter 2-1 (works best becuase it has the shortest wait time before playing)
4. As soon as you can, start split-screen co-op
5. On the item management screen move Chris's egg to Sheva.
6. When you get back to the game, have Sheva give the egg back to Chris.
7. Exit the game. Save Chris's item data, but do NOT save Sheva's.
8. Repeat from step 3 until you have 45 rotten eggs (the max you can carry) on Chris,which can be sold for $90,000
9. After this you can keep giving the 45 eggs on Sheva over to Chris as much as you like as long as you never save Sheva's item data.
What is happening is that all of the eggs that player 2 (sheva) gives to player 1 (chris) are being saved onto Chris's item data, and when you do not save Sheva's data she keeps the eggs she was carrying, allowing you to keep sending eggs over to Chris infinitely.