A friend ask me to post something here on ebay to protect buyers, some here are some ideas just off the top of my head.

Art is very subjective, eye of the beholder, if you buying from the original artist, buy what you like and what feels reasonable $$$.
But buying an investment is very different, its not so much the art as who painted it, the proofs of the artists actual connection to the piece.

You want to look at the front and back.
If the canvas is stapled on the back of a 1800's piece of art something is wrong.
Look at the patina of the wood, is it old how is the frame put together, what kind of nails, hangar, etc.

The painting its self also has a patina :dirt dust of the ages, texture, sometimes, the brightness of colors, or lack there of.

Research the piece, know the artists work, look at many of his pieces, compare signatures, get close ups of the sig,
Check for a match.

How does it "FEEL" to you. Balance everything, many fakes for example have no depth of paint, layers, its easy to copy a piece electronically but a photo copier cannot get the layers of paint a real piece has.

Study the canvas, do a thread count, modern or old? Is there some patina on the back surface of the canvas?

A fake painted copy needs everything else to match, frame wise, and a patina is hard to reproduce, visit museums study the patinas, ( ask to see the back, they will show you ) the feel and look of old art works, study the depth, layers of paint needed to achieve the color desired by the artist..
Painted copies sometimes will have hairs from the cheap paint brush still in the paint on the canvas.

When you do get your hands on the painting SMELL IT, IT TAKES OIL A WHILE TO DRY,  years to completely loose the smell of oil.

If it is something you are in LOVE with, you need a third party to independently review it, someone who is not in love,
eye of the beholder kind of thing.

What have other works of this artist sold for at other auction houses, this size, time frame, and same medium.

There are art galleries on ships selling art works for very reduced prices, ask your self if it is worth $10,000 are they selling it to you for $4,000.
They confuse the buyer with sizes and periods, even mediums to sell a lesser piece at inflated prices.

For prints they must be signed, and numbered.
Signed in stone is of little interest, ie unlimited copies can be drawn.

Many pieces will have gallery stickers or written on the back, research that gallery, have they been involved in any funny business.

There should be some signs of wear, on the frame, even the canvas sometimes. wooden edges not quite as sharp after 50, 100 years, and drier.

Research the artist has HE been involved in any funny business, some artists are known to have signed blank paper, which later have prints drawn on/from them,  which means he did not even supervise the pulls.  These would be of significant lesser value, Salvador Dali was know to have done this,

There is a new scam where the print is not numbered but another document is, which is meaningless, as any signed in stone can be used in place of the real item.

Background:
Experience
Directed Antique store for 10 yrs. Diamond broker 7 years, directed auctions 3 years. Curator Museum 3 years Chairman Artifacts Committee, Museum I have sources in Europe, England,Central and S. America and N.Africa, as well as sources in most large auction houses. Archaeologist trained at Mich. State University, Field study: Marlem Tarn. England. Appraising art and antiques for 10 years.

I hacve a  web page for cell phones, where you can call me from the field for help, send photos and ask questions,