Anything made largely of wood is likely to degrade with time. Guitars are usually made of a variety of woods combined, some very thin, and this heightens the possibility of warping and splitting. Varnishes are likely to be buffed or chippied. Not to mention the electronics, which can be scratchy, faulty or just plain broken.
If you are going to buy a guitar online - and it can be done easily and safely - insist upon the vendor publicly stating on e-bay that they are offered a money-back gaurantee of at least a week from the date of delivery to the date of your return postage receipt.
Otherwise, you run a very high risk of being very disappointed.