Quality and condition, always buy the best affordable. Never buy swords that have been disassembled, they can be bits of this and that making it valueless.
First have a good idea what your area of collecting will be. Swords with provenance and or regimental markings have much more history connected directly to them.
Cavalry, infantry, naval swords, either troopers swords or officer swords. Actual combat swords or dress swords, the list goes on.
Always buy swords with their original scabbard, trying to find a scabbard later is next to impossible and the variation in size from maker to maker does not allow for a good fit. The blade is the most valuable part, a shortened or broken/repaired blade is next to worthless unless extremely rare.
Cheap bad condition swords never gain value and are difficult to sell. Swords are an investment so always keep in mind resale. At some point your area of collecting may change and selling some swords to purchase others now in your field of collecting.
Purchase reference material such as sword books, learning as much as possible will save you money and well worth the investment of a few hundred dollars in books.
forums such as Sword Forum International, the members share much valuable information. very useful in researching a sword.
Most collectors graduate to swords regimentally marked or officers swords with initials, family crests on the blades. Wilkinson swords usually have a serial number that can be researched for a small fee, done quickly over the web. http://www.armsresearch.co.uk/
More to follow