The concept of "trade-in" is pretty simple Lindy. Say you want to buy a new car but you don't have or don't want to pay all that money to afford it. And say the car salesman really wants to sell some cars (he needs to sell cars to make money after all). The solution is that the car dealership pays you money to buy your old car (used piece of junk for example), in exchange for which you buy a nice fancy new car from the dealer. Everybody's happy---you were able to afford a nice new car, you got rid of your old car, and the dealer is happy because he got rid of the new car and made some money.
As for the MD87's...they are too small and they are not 100% parts compatible with the rest of the MD82/83's. Either get rid of 260 MD83's or get rid of 5 MD87's. You decide.