Never ever buy a sword online or anywhere else without going to
www.swordforum.com and doing your homework.
Once you register on that message board you can search and ask in the beginner's section. This is HANDS DOWN the best repository for sword related information the web.
As for reliable places to buy swords, it depends a LOT on what you want.
If you want a wall hanger or slo ('sword like object' is also referred to as a wall hanger, this refers to things that look like swords but were never, ever meant to be swung at anything), then you are basically just looking for a reliable vendor. Check for street addresses instead of P.O. Boxes. Use a credit (not a debit) card which affords you buyer's protection (the change to reverse the charges if you are ripped off).
If you want a real sword then decide if you want a production sword from a reliable company ($200-$1000) or if you want a custom sword ($1000 - ?? and a several month to several year wait).
If you are going to go with the first choice (good production sword from a reliable company). Then I would seriously consider going with an Angus Trim sword. These are swords made from a stock removal process (machined) but designed and well made to be optimal cutting swords with traditional designs and styling. They run around $300-400...and are very, very nice blades.
Let me know if you have any other specific questions...I could go on for a long time.
Some other reliable companies are ...
http://www.armor.com
http://www.albionarmorers.com/
http://museumreplicas.com
http://allsaintsblades.com
...and many more. Although the ones I listed are more targeted at Western Style swords.
Cedric