Goal: to get the PCs to spend their time in the field finding items rather than in the lab building them.
Way to achieve goal: make creation of any item other than a simple potion or scroll take time counted in large numbers of months if not years. Most PCs won't want to take that long of a break, particularly if the rest of the party keeps going and-or the plot runs amok while the party waits. That said, if a PC wants to commission the construction of an item now and come back for it in 6 months or a year or whatever, that's fine.
Goal: to allow some buying-selling-trading of magic items without having Wal-Magics in every town.
Way to achieve goal: assume that most items for sale are coming from other parties just like yours who have - like you - found something they don't need and-or can't use. (though temples and MU guilds might have a few potions to sell.) Barter or sales are usually carried out at guildhouses, meeting halls, pubs, etc., rather than in a streetfront store. What is available to purchase at any given time is almost completely random.
Lanefan