There is a very good guide written by Greyman2.  However, there is one other total giveaway as to whether the sword is valuable or not.  In the making of the sword, after the folding and shaping, the whole thing gets put back into the forge to temper the steel.  In order to get an edge that can be sharpened with a back of the blade which can absorb impact, the swordmaker will cover the "live" edge of the sword with clay befpre the tempering.  This gives a non-symetrical wavy line of hard steel in the metal that is very hard to reproduce and is easily visible.  If you see that this area of the blade is in a neat pattern or ground into the blade during sharpening, then you have a mass produced item.

You cannot go by the Tsuba (guard) or by the hilt as all the sword furniture is intended to be replaceable - indeed they would be changed as fashion dictated.  Only the blade and tang really matter.  The blade should have an unsymetrical wavy line, the tang should have a makers mark stamped into it.  The katana should also be more curved than not.  Don't go for so called "ninja" blades.

It is very unlikely that a real quality sword will come up on e-bay, and no-one who knows about them would rely on just a photo.  By all means get a simple example of the pattern, but don't expect more than mediocre quality.  The best are considered to be works of art and are treated as such.