There is some debate as to if spells one writes themselves are better then ones that are purchased or pre-written. Many traditionalist witches insist that one must write their own spells for them to be most effective. While I strongly encourage writing one's own spells, I recognize that sometimes it is simply not an option due to time constraints, availability of resources, or other unavoidable limitations.
It is important to remember that purchasing spells from another is not a bad thing. It can be an economical option. There is also a great deal of historical precedent set for purchasing a spell or contracting the services of another to perform said spell. That said, let us look at the costs and benefits of acquiring a spell that is crafted by another person.
The first and most obvious benefit is that the work of developing the spell was done by another person. The spell has been built upon established systems of correspondences and magical associations, requiring no research on your part. Presumably the spell has been tested before being set out for purchase, thus has been proven to be effective to some extent. Also, the list of supplies required for said spell is established and thus easier to locate.
It is more expensive to purchase a spell rather then write one. This does not always include the material components for a given spell. It does not resonate as strongly with one's personal energies and goals because it has a very general focus. A purchased spell is also difficult to customize because efforts to customize the spell may have unplanned for effects in how the spell manifests.
Writing spells for oneself allows one to tailor the spell to ones personal energy and specific need. It is significantly less expensive. Also, writing your own spells allows you to work within your personal spiritual identifications and correlations. Crafting your own spells allows you to use whatever you happen to have on hand at the time, which can be an additional source of savings.
They do, however, require some research to make the spiritual connections of the spell strong. It also is difficult to predict how it will manifest the first time you cast it.