True, you only learn through making mistakes. Educations are rarely free! I'm going to educate myself more before I go after stuff that's commonly tourist like kerises and koummyas. It's sometimes almost impossible to judge the authenticity of something off a blurry ebay photo. I think if you ever want to build a respectable collection you have to spend the extra money and buy from a reputable seller.
Here's my advice for newbies looking to get a deal off e-bay:
Beware any knife/sword with a two dimensional looking blade. If it looks like a flat piece of metal cut into the shape of a blade, it's probably not made for actual use. Even if the blade is well decorated, you should see some evidence that thought was put into how the blade would cut. Look for things like fullers, ridges or convex edges.
Beware any knife/sword with a blade that still shows a lot of marks from hammer forging. If the hilt and scabbard look nice but the blade looks very roughly finished, it was probably never meant to be seen IE it's a costume piece or a wall hanger. I used to think a rough blade meant it was made for "real use". It really means it was made by someone with no skill out of scrap metal. If it was real, even if it's for a poor person, they would have at least put the effort of polishing the hammer marks out of it. Working with soft metals like brass and copper and wood to make a nice hilt and scabbard. Making a steel blade is difficult, and if it's never going to be used they just skip that part.
If the blade doesn't look right but you find yourself staring at it until you convince yourself it's legit, it probably isn't.
If there are any unusual markings or writing on the blade, type it into google and see what comes up. If you find 10 identical blades it's probably mass produced for tourists.
Don't buy anything from your cell phone before you look at it on a larger screen. Especially if you're at a bar. Especially if it's exactly what you were looking for and the price is too good to be true and nobody's bid on it.
Avoid knives/swords that are commonly made for tourists unless the pictures are really good and you know what you're doing. Moroccan kouymmas are like that, you see 10 tourist ones for every 1 real one. Usually the blades on the tourist ones are terrible looking, though, so they're easy to spot.
If you're looking for a blade from a certain country, don't buy one from somebody in that country. It's probably a tourist piece. Find out who that country's former colonial master was and look to buy it from somebody in that country.
100+ year old steel ages differently than more modern steel's. Learn to tell the difference between the patina of a centuries old piece of steel that's been well cared for versus a 50 year old rusty piece of steel.
Do not believe anything the seller tells you, only believe your eyes.
If the seller says it was brought back during WW 2, then it's probably a tourist piece sold to a GI.