Everyone will tell you this. Everyone ends up ignoring it.
The Big rule #2 is to do your homework before you start buying. This is generally refined down to "buy the book before you buy the item." Meaning, read some books on the subject before you start picking things up.
If you don't you WILL end up buying stuff that is junk, or overpriced, or fake. Then you'll eventually find out about it and become bummed. IN some cases you will then feel so crappy you will lose the joy of collecting. This has become a major problem in collecting German WW2 items, because 98% of everything sold in this field is fake. Or maybe 99.5%.
Some of it is so good that unless you've been handling it for years, and have stayed up to date on what is going on in the field you just can't tell. And of course there are dome really evil, shitty, hell bound people that write books on the subject that knowingly show a fake as real- so as to enable them to sell junk. A friend of mine was once threatened with physical violence for writing articles on how to spot the fakes. Another was threatened with an advertiser boycott of a publication because the magazine printed such articles. I'm told that certain major dealers in Britain that drive really fancy cars have hired goons to deal with this issue.
More beginning collectors have started out by buying a few items, generally not that cheap, and when they find they have been screwed, they stop collecting. I used to predict this would really hurt the market as a lot of people who might have stayed in the hobby would leave, and thus remove a lot of customers, but there is a sucker born every minute- and two when it comes buying Nazi stuff.
Mea Culpa. I've been there. You see something you did not know you could actually get, and your eyes become too big for your brain, and you plunk down good money assuming this is a bargain, or it is a once in a lifetime chance, or that you cannot wait one more minute to start in on this new hobby. I'm pretty good at avoiding this, but to my shame I will tell you the last time I was took.
I discovered you could buy inexpensive roan coins that you got to clean, and figure out where they came from and what time period. No problem there. I read enough before spending money on some. I got my money's worth and had a lot of fun and learned a lot. I still dabble in ancient coins when I need a diversion. But I was suckered by a similar area.
I spotted on ebay a guy selling roman coins that also had some roman artifacts from an "ancient battlefield." Now being really interested in the Roman Army I HAD to get some before this great opportunity went away. The dealer was a bit cagey in his answers, but I fell for it. I sent (not a large sum of money, but probably a good 3-4 times what they were actually worth. As I learned more I found that the same kind of old metal junk is found all over the place in Europe, and I probably could have bought a whole bag of odd bits that were similar for a lot less. I fell for his description of them coming from a field where Roman battles Germanic tribesmen. Great story- but almost certainly made up as it sounded cooler than "old metal bits that look Roman in time period that come from the leftover detector finds that no one wanted." I still have them I a case to remind me not to jump on something before doing my homework.
So, two days ago I stumbled on something that I thought would be kind of fun, but I did not buy. I read. I read all the ebay listings., I searched the web for collector's groups, I read the 'how to' web pages kind people had put up. I learned a lot. In the end I went into it with eyes wide open and buying material from a reputable source instead of a "get rich quick" ebay junkseller. I'd say what it is but it is part of a cunning plan for a gift for someone that I do not want to tip off.
And the Corollary to this rule is "do not trust dealers." If someone is making money by selling you something, they are tainted. Now, many dealers are honest, but many are not. They will sing you a song about how this button was only worn by the special guards at Abraham Lincoln's funeral, or that this doll arm is the hardest part to find on a 10" Shirley Temple. Anything to make the sale.
Just say no, until you have really looked at the hobby and fell you have a handle on it. Ebay makes this so much easier. Case in point. There are 10 million unissued WW2 era US rifle grenade sight kits out there. There is no proof at all they were ever even used in the war. Everyone has them for sale, and every collector probably bought one early in his collecting days as he figured it was a cool item, but they are pretty much worthless (OK, so maybe $5). Now in the old days if you went to a gun show and saw one for $30 you might think it was hard to get and snap it up. Today, a simple search on ebay will show 10-20 listed at any one time. Some for very low prices, some for very high.
Love or hate ebay- it can give you an instant snapshot of a collector's field from the comfort of your own home. Don't trust all the wacky stories tell you (found in the attic of Abe Lincoln's tailor's grandson), but use it to get a quick idea of what it out there. If there are more than 10 of something on ebay at one time, it's probably not all that rare.