Characters who can cast spells will find that they can do so and make a pretty decent amount of money in their spare time. This system applies to cities in Attellaria, but you will find the government in most places will have the same requirements (although some will prohibit this activity entirely).
First, anyone wishing to sell spells must purchase a permit, which they must show to any prospective buyer. This permit costs 100g, and is good for seven days. In addition, the seller must pay a tax of ten percent of any income made from selling spells. A person doing this will be able to sell d4-1 spells per day, with most of these spells being no higher than level three. If a character is selling spells, roll the level as a d3; if a 3 is rolled, roll again, and if a second three is rolled, count it as a level 4 spell (and increase the income), and roll again, until missed.
The penalty for selling spells, or buying spells, without the seller having a permit is a fairly stiff fine, calculated as the most that a seller could probably have made during each day of such selling. Estimates tend to be generous, in both the level and duration. Divination is typically used to find out what level the caster is.
If you wish to sell spells for a prolonged period, you can expect to sell 1.5 spells per day, making this much money (base) per day. Note that the vast majority of spells sold will be levels 1 to 3, with the occasional level 4 and very rare level 5, so income does not go up much after level 6.
If you wish to hire an NPC to cast spells for an extended time, they will charge the above amounts per day, plus enough to cover their weekly fee and taxes, plus all material components. NPCs over level 10 who cast spells for pay are very rare, and must be dealt with individually. The same applies if you desire to pay an NPC to inscribe scrolls; they will charge for their time, plus expenses above.