When buying art on ebay it is very important to check the information for details such as size and weight and general dimensions. If the colour/s are an important factor be sure that the images displayed are of good resolution and colour match. If not entirely sure write to the artist and ask them - they won't mind. As makers of visual objects we understand the importance of their setting and surroundings and it is our responsibility to ensure that the item for sale is described fully and represented clearly and accurately. -- It is possibly more important for this particular catagory than any other.
While it's nice to see art in the flesh before buying, there are several advantages to using ebay, so long as you have followed the advice above. The range of styles and media is extensive and many artists on ebay can not be found anywhere else, and may even live in another country where you will probably never have found or heard about them. There's also the advantage of being able to take your time over finding what you like without having to make several trips to several shops or galleries and being restricted by what's available in your local area.
There's not much that's better than getting a parcel delivered that has something inside that exists purely for its own sake. So... enjoy!