Since most art is purchased for display purposes, probably the most important point to consider is whether you like the picture or not.  Coming right behind that is where you intend to display the picture and how it will fit in with your decor.   For example, if your furnishings are traditional, it would not be a good idea to display contemporary abstract art in the same room.  Another consideration is the specific process used to create the art.  Original Paintings are the most unique and usually the most expensive.  There are many self-represented artists who list on eBay and this affords an individual an opportunity to acquire original art at what are usually low prices.  Some original paintings are copies of more famous artists' work and the quality of these varies.  You usually get what you pay for.  Published editions, usually consisting of a limited number of identical pieces, each signed and numbered by the artist, though more common than original art, are still unique and can be of value.  Such editions are produced as Original Serigraphs, Original Lithographs, Giclees, etc.   There are some very great values in these limited editions and eBay is an excellent resource for these types of Art.  Less expensive art consists of open editions, lithographic prints and posters.  Framed nicely, most individuals cannot discern the difference between these and more valuable art unless they come close to look for the artists' signature and the edition numbers.  Of course, there is much more to know than what has been said here, but this is a start.  Remember, most important for most purchasers is whether or not you like the picture.  Check out our store at ArtzDirect Fine Art LLC.