First tier airlines like to have new planes with all the latest features. Second tier cheap airlines will often buy a used plane: when that money is applied to the cost of a new plane is cheaper (or the same cost) as refurbishing the old one. From there third tier airlines exist that will buy planes past their safe service life and pray they don't crash.
What would be an example of a “third tier” airline?
Usually cargo or charter operators in Africa, or the like.
The first ones off the production run aren't as fuel-efficient as the later ones. I read somewhere the first few A380s are several tons heavier than further down the line.
That doesn't explain why one wants to sell and another wants to buy the plane.