Other than high price, a novice won't learn much from just buying a finished tree, and might end up killing it with lack of hands-on experience.
Believe me, most of the enjoyment in having a bonsai is bringing it up to a finished tree. Pruning, wiring, repotting, watering, fertilizing, and watching it change over the seasons is probably the most enjoyable experience than just having a bonsai.
If you want an instant bonsai for instant gratification then buy a bonsai, but if you want a lasting and a growing gratification of bonsai, then enjoy the beauty of it by working on nursery plants, wild raw material, or by air-layering.
- AH