3 IMPORTANT FACTORS
1-MAKE SURE THE PAINTING DISPLAYED IS THE PAINTING YOU ARE BUYING
Sometimes the seller will sell an item that has not yet been painted. They show you a picture of the painting you think you are buying. If you read carefully, you find that the picture is not yet painted.
2-FINISHED VS. HAND DRAWN
Beware when buying a canvas painting. Make sure that you are actually getting an original and not a hand finished print.
These items are often sold as "hand painted" but are in fact printed and brush strokes added to make the painting appear genuine. This hand finished print could technically let them be called "hand painted", but are in fact only crude reproductions. Country of origin can give you a clue to this as can the amount of reviews they have.
Make sure you know what you are buying by asking the seller and looking at their eBay reviews.
This can be a clue to an item being a hand finished print. Most real artist will normally stretch the canvas before painting, but a print will be stretched afterwards. But more importantly getting a canvas stretched or framed can cost a great deal of money, often far more than would be saved on the shipping cost. Make sure the items you are buying come stretched. Paying extra for a stretching service can be expensive and time consuming.