Just to lay out the whole range of what you will find in a reord store.
Single --> Maxi single --> EP --> LP --> 2LP --> 3LP...etc. --> Box Sets.
Promos, For sale promos (those $.99 and $1.99 single cds are actually free to most stores), cutouts (no longer made or distributed), and consignment (usually local acts on a 30% to 50%contract with the store).
From what i've come to understand... Extend play albums started off as being extended maxi singles meant to test the waters early on new groups without putting out a whole album.
I know some shit about the pricing to.