Basically, low-level magic items, like +1 weapons and armor, are fairly available; players in my campaigns regularily sell excess magical weapons if no one in the party has a need for them.
Scrolls are generally kept by whomever can use them. Potions are as well. Wands are almost never sold, because again, someone in the party can usually use them.
Miscellaneous items are sometimes sold, but most often kept by the party.
As a result, a player in one of my campaigns could go out and buy a +1 item, usually only having to wait a little while, and paying typically about 5%-10% above list unless they bargained really well. Miscellaneous items are much harder to find. Powerful items almost never are bought or sold.
Scrolls are definitely not easily available. If a character wanted a cleric scroll, he'd have to go to a major temple of the deity that character worships, and pay for it, usually at or above 'list'. 'Clerical' Potions are more easily available, things like Cure Light potions or sometimes Cure Disease. These are naturally available only to the devout of that religion, and they're available at list price, but only in limited quantities.
A magic-user can be hired to write magic-user scrolls of 'common' spells, or create magic-user type potions, but the cost is well above list. Mages in my campaigns tend to be the suspicious/secretive type who are extremely careful about what they give out.