Guitars, like any other instrument constructed out of wood are very susceptible to changes and/or damage from improper storage. Humidity and temperature levels need to be held in an optimum range to make sure there is not opportunity for shrinking or cracking of the wood, glue joint failure, neck twisting, and other issues.
Additionally guitars should be adjusted and setup before shipping as many of the final adjustments are not made by the manufacturer. Only knowledgable and skilled dealers and technicians would be able to perform these tasks. This is not something that Amazon as a warehouse 'distributor' does on their own product or any Fulfilled By Amazon (FBA) products.
As stated above guitars need adjustment and setup services performed before being sent or given to the end user. This service if not done when the guitar is shipped would need to be performed at a qualified service facility and would add to the expense of owning the instrument as well as the hassle of searching out and taking the instrument to a service shop.
Dealers who provide this service when selling on Amazon are adding considerable value to the instrument by doing so. Unfortunately due to the way Amazon listing are categorized and organized it is impossible for individual seller to identify their offering of this service. Listings are more generic and specific to product features and the dealer listed in the 'Buy Box' is not sorted or given priority by any qualification for providing this service. In fact because the 'Buy Box' is almost solely driven by pricing and volume, there is no correlation to additional service provided.
The guitar, like any other instrument is a precision work of craftsmanship when constructed properly. The geometry needs to be precise to play properly and the construction needs to be proper to withstand the string tension and other forces being exerted on the materials.
The selection of materials including woods and components like tuning machines dramatically affects the sound quality, playability, and longevity of the instrument. All of these factors will also affect the experience of the player and whether or not the process of learning is fun or frustrating.
There are a lot of guitars being sold that actually cannot be played as a true instrument due to faulty design or improper construction. Because of the wide appeal and high levels of shopping traffic Amazon has a large volume of questionable instruments being listed. As always buyers should do their homework with some general skepticism about pricing that seems too good to be true or drastically out of line with the pricing for known reputable brands.
An instrument that has been designed to play and tune properly may cost a little more money but will allow the player to enjoy the process of learning without the frustration of performing the right actions and hearing a less than desirable result. That type of situation is no fun for anyone and pretty much guarantees that the instrument will not be used. Sadly this can also result in the potential player giving up the idea of playing music.
It is wise to find a reputable dealer within the listing on Amazon and be particularly aware that Amazon as a seller is essentially an efficient warehousing and shipping entity for 'boxes'. There is no provisions in their operations for optimizing the storage, distribution, or service of guitars. Dealers who are listing their items for sell on Amazon and fulfilling their own orders can control these factors if it is part of their business practices. For this reason if you are determined to buy a guitar on Amazon look for a dealer who fits this criteria to get the absolute best value for your money.
In summary if you are going to purchase a guitar on Amazon it is wise to look through the different sellers and do a little research on their background and experience. Making sure they are a musical instrument specialty seller and have the knowledge and experience to store the instrument properly and perform the necessary setup and adjustment work before shipping the instrument is vital to getting the best value for your dollars spent.