I bought my last 2 bows on Ebay and havent had a problem. Once I had a budget in mind, I researched 2 or 3 of the bows I wanted (the ONE you choose, may not be on Ebay). I came up with a price they should be valued at based on their condition and went to bidding. I wanted mine stripped so I could accessorize it to my liking...as many used bows come all set up "as is" on Ebay. I saved a significant amount of money as opposed to buying a similar model bow from a pro shop. I purchased and put on all my accessories seperately and changed the string. I still shoot my Hoyt that is 15 years old...its good as new.
One good thing about doing this in the spring is that you have time to research and purchase the bow and won't feel pressured into getting something in haste because bow season isnt right around the corner.
If you dont know how to measure someone for a bow...also, if she hasnt really ever shot a bow, its important to know what draw weight and style bow is comfortable for her...as you know, this is important. Is it something she wants to persue for years or something she just wants to "try" to see if she likes? A less expensive set-up may be the way to go then. It may be best to go to a pro shop and have it done...you may even find a good deal there...but IMO, Ebay is cheaper, if you know what your getting. Many people like to see what they are buying when it comes to a bow, but Ebay is pretty reliable and I found both my sellers to be knowledgable and honest. You can email the seller questions and you are protected by paypal.
My apologies...I assumed this would be for hunting, but it may just be for recreational shooting...either way, nothing should be different.