Choosing Art is a very personal exercise. What floats one person's boat is hideous to another. Important things to consider are:
Size - make sure it will fit in your designated space before you buy. Pictures of art in home settings displayed on auctions are often misrepresentative of the size of the artwork. It may look like it covers an entire area above a sofa when it really is much smaller.
Oil or Acrylic? - When I used to think of art I thought Oil. But since I have become an artist.. I think acrylic. Acrylic is a much faster medium for the artist as you can paint and sell all in one day. Oils take up to a year to cure. (dry properly). If you are buying oils make sure they have properly cured and have been properly sealed, especially if you are paying a lot of money for an investment piece. A lot of decor paintings are fine to have in acrylic or even multi media styles. They will last and be around for a long time for you to enjoy. That is really what art is all about.
Framed or Gallery Stretched? - Personal Choice, You can always frame a piece at a later time if you choose. In other words, those paintings that come 'ready to hang' stretched on a wood frame can be inserted by a framer into a professional wall frame and they look awesome! But until you are ready to frame you can still hang and enjoy your artwork