In 1E or OSRIC, there is a general attitude by many players and DM's that one should not be able to just walk into town and buy a magical item like a scroll or a ring, staff, sword, etc... As later editions of AD&D seem to be more permissive of.
I tend to take the Goodkind approach (Terry Goodkind, author of the "Sword of Truth" series) in that apprentices and and "retired" magic users would be found selling scrolls and enchanted items out of their home or shops to make a living if they were no longer able to "make a living" by adventuring.
I have no problem with this. It makes sense to me. I would say that the chances for more advanced magic decreases the higher level the spell or enchantment is.
For general purposes, I think that magic up to 4th level would be fairly common. 5th to 7th level would be uncommon, 8th to 10th level rare and above 10th would be very rare and/or very expensive.
When it comes to pricing of said items, I kind of went with a pretty simple formula.. For example...
Level 1 magic = 100 g.p. per level, Level 2 = 200 g.p. and so on till level 5.
Level 5 magic = 1000 g.p, per level through level 7. (ie... level 6 = 2000, level 7= 3000)
Level 8 magic = 10,000 gp per level through level 10 (ie... level 9 = 20,000, etc...)
I won't allow for the commercial sale of magic above level 10. That level of magic is either gained by adventure/fought for/stolen or is given specifically to the PC by a magical entity.
Using percentile dice, I figure it that magic from level 1 to 5 would be by far the most common found, so if rolling to see what magic is available when encountering said magic selling person...
1 to 50 = magic level 1 to 5 only avalable.
51 to 67 = magic levels 1 to 6 will be available.
68 to 85 = magic levels 1 to 7 will be available.
86 to 90 = magic levels 1 to 8 will be available.
91 to 95 = magic levels 1 to 9 will be available.
96 to 100 = magic levels 1 to 10 will be available.
Of course, no magic seller will have every magic item to sell. They will be limited to what they do best at, what is easiest to make/procure given their assets and abilities, etc..
Most of what they carry in their shops, depending on location again, will be geared to the population around them. Love potions, cures, etc... will likely be their largest stock.
Magic vendors in large cities will likely have items more geared toward fighting and adventuring than small towns and villages who only see fighters/adventurers on a sparse basis.
Also, not every town/city will have a magic seller. Then again, some cities may have more than one.
Mind you, this is just how I do things at my table, not a suggestion that others do the same or that game systems in general should be changed.
I know of one Magic User who in their adventuring "off-time" will wander from town to town, offering to enchant items or make scrolls to be sold in local venders shops for a price. This is how he continues to gain revenue to pay for the many items like material components, thieves, etc that he employs in his own pursuit of magic.