There a number of key points when purchasing a sword which I will go through.
Steel Quality
The first point is the quality and type of steel
used in the blade.
If its stainless steel, its a decorative sword,
nothing more. It doesn't matter where the steel comes
from, stainless steel looks great, but can
not
handle the punishment of being used.
Generally speaking, the sword should definitely be a
made of high-carbon steel. For a sword that can take as
good as it gets 5160 steel is the choice. Its used in
truck springs, and is very strong. Also, ensure that
the quality of the steel is good. Recycled steel can
be good, in fact some of the best swords are made from
recycled steel.
However, some recycled steel is not recycled correctly,
and can be of very low quality, causing micro cracks to
appear, reducing the overall strength of the sword.
You should talk to the seller about where the steel
came from, and what type it is.
Heat treating
An often overlooked attribute when looking for the
sword, how the sword was made is very important.
Heat treating is designed to create a balance between
the toughness and hardness. Toughness defines the
sword's impact resistance and shock handling abilities,
while hardness defines the sword's ability to hold an
edge. If the sword is too hard, it will be very sharp,
but will shatter or crack when hit. If the sword is too tough,
it will dull and get nicks in it.
The Rockwell "C" scale is a measurement of the hardness of steel. If you are purchasing a sword from someone who has never heard of this scale, then perhaps you had better think twice about the quality of the sword they are offering.
40 HRc would be the minimum on the Rockwell scale I would recommend. At this level, the sword will be springy, but shouldn't chip if hit edge on edge and dents can be hammered out. Anything over 60HRc is getting dangerously brittle. 60HRc is where the high quality Japanese
katanas lie. At 60HRc, edge-on-edge fighting should NEVER
be done, since this is not the correct use of this type
of sword, and will defiantly cause chips.
Handling
This is something that most of us, if aware of
how a sword should work (eg generally not be so damn
heavy you need all your strength to wield a one
handed sword) should be able to detect reasonably well.
A sword's optimum point of balance (or PoB) varies with the type of sword, however the usual optimum spot would be between 3 to 6 inches from the guard. If the sword is handle heavy, then the sword will feel light to wield, however it will have little effect when it strikes a target. On the other side, a sword that is blade heavy (very easy to detect), will have great effect if it strikes, however it will be very tiresome to swing around.
Some cheaper swords achieve are inherently blade
heavy, and combat this by increasing the weight of the
pummel and or increasing the length of the hilt.
This can be noted by looking at the overall weight
of the sword, and the length of the hilt.
Some swords have a fuller that runs from 2/3s
to the full length of the blade. This reduces the
weight of the sword, but note that a full fuller
*could* weaken the blade at the point, this is a matter
that is hard to judge unless you see the sword yourself.
The sweet spot of the blade is the spot where, if hit, vibrates the least. This is the optimum point to strike with at your target. A poorly designed sword will have no sweet spot, or one which isn't located near the end of the sword. All swords vibrate when hit, if these vibrations are large enough, they can transfer to the hilt, and make holding the sword in battle difficult.
The tang
My favourite part of the sword, because it is often
overlooked and forgotten about, until it breaks.
The tang is the part of the blade which extents
into the hilt (or handle). A badly designed,
and practically useless sword will have only a half tang,
meaning it doesn't run the entire length of the hilt.
Do NOT buy a sword that has only a half tang,
unless you are only looking for a decorative sword.
(Note: I was just informed that traditional
Japanese swords only have a half-tang since they are only
used for slashing. Which proves a point that you should
never just listen to one persons opinion on what to look
for in a sword)
In my opinion, the best tang is the rat-tail tang,
where the pommel is screwed onto it.
This means the pommel, and with it the entire handle,
can be removed. This allows for inspection of the tang for weakness or (heaven forbid, rust).
Ideally, the tang should taper from about 3/4 or 2/3s of
the width of the sword, down to a thin point the pommel
attaches to. If the tang is a simple thin point straight
from the handle, with no tapering, then this becomes a
weak spot, which can snap during battle.
The other item I will include in here is the hilt. Some cheaper hilts have a circular hole which the tang enters. However the tang is not circular, this leads to loose hilts. One way of spotting this is to sharply rotate the sword about the long axis, and note if the hilt moves at all.
In all, if you think about it, a lot of the points (pun intended) of good swords are easily guessed and can be looked for, but beware of the less obvious, and sometimes invisible attributes, such as the tang, PoB and sweet spot.
There are numerous resources available on the web and in books that go into much more detail about purchasing swords, and if you are purchasing a particular type of sword it is best to read or speak with experts on that type, in order to understand some of the unique points for that type of sword.