Many years ago I would really not know what to buy. Just buying a nice front cover did not help.
Then I subscribed to JAZZIZ magazine, which once a month sends me the magazine and a CD with a selection of different Jazz artists. Their CD´s are very high quality, and from what you hear you can go out and explore those artists other albums. In Amazon you can listen to them and decide.
I also listen a lot to SMOOTHJAZZ.COM while sitting at my computer and allthough quite repetitve often I come across some new jazz which I like and instantly order through a link on their web page.
I discovered new artists and new music through these methods and now my jazz collection numbers almost 500 CD´s.