What are some unwritten rules about watch collecting?

Fantastic advice.  Also buy within your means.  This hobby is super addictive and can get out of hand.  

That there are no rules.

Budgetary: Research, research, research. 

Style: Research, research, research

Specs: Research, research, research. 

Buying: Hang the rules & buy what you like. 

Edit: One ACTUAL rule I do go by and that is I never buy a watch on credit if it will cost me more than purchasing it outright. 

There are no rules, just get what you like.

Thou shalt not sell!

Do not lick lume

Don't start.

Don't write anything down.

Don't online-order under the influence.

Half the fun is making the mistakes, I’ve made them all but learned whilst enjoying the anticipation of yet another watch. 
What I have learned which should have dawned on me much earlier is that your wrist size doesn’t change so if a certain size watch (and it’s the lug to lug which really matters) is too big it’s always going to be too big. Other than that prepare yourself for the ride……….

Wear a G-Shock in a suit.

Wear a JLC cleaning out the horse barn. 

That comment is highly underrated.

Rule 1: You don’t talk about  Fight Club!

Wait, that’s something else. If I can echo the sentiment of others, live within your means, get what you like, research, and there really are no rules.

Above all, have fun!

As a senior collector of two years (lol)  I will paraphrase what IceCreamMan said.  Truly, no one except someone who is into watches will know what you are wearing and would not care.  If you really want to get attention and people stopping you, wear a Concert Tee or a Shrek Tee shirt.  Every time i wear a Kiss concert tee or my Shrek tee, I get several comments about how cool my shirt is.  lol

If I had to do one thing different, I will say patience before going on a buying spree.  It is easy to get caught up in this addiction and buy way too many before you realize what kind of collector you want to be.  I guess it's all part of the learning curve.  So I would say if you can, practice patience and obtain more knowledge.

Only one rule: Don't be stupid 🤣 (don't overspend basically)

1. Never buy a watch, because you´ve been told you need it. Only buy what you love.
2. Try a watch on, don´t care about the size in numbers
3. Never buy on credit or overshoot your budget, you feel comfortable with.
4. If thinking about a new watch, take much time to check alternatives and if you really want it. Think about when you want to wear it. Money is only wasted if the watch is always in the box.
5.  Don´t compare specs when deciding betwenn diffenrent watches. You won´t love a watch for 100m more water resistance etc. 
6.  When you bought a watch, that makes you smile every time you look on your wrist, you didn´t made a mistake. Don´t listen to others claiming, there a better alternatives or nicer watches. It´s only about your joy.

I broke all those rules and regret it, but thats part of this hobby. You will always learn and your taste and opinion will grow with time. Always look back with a smile. 

1. Buy what you want and not what you need. Unless is both wants and needs.. LOL

2. Always wear watches on 2 wrists. Balance out the weight.

3. Don’t buy into the hype.

4. Quartz is just as good as mechanical and automatic watches. 

5. Small watches are not girly. Big watches are not manly.

6. There are no watches specifically for guys or girls. They are unisex.

Wear any watch with enough confidence and you'll start a trend.

Very much not my rules, but what I see mainstream watch attitude is. Beginners should follow these if they want to be accepted in the community.

1. 1970's styled watches with integrated bracelets are the coolest things around. Paying over €100 k for a steel  Nautilus is perfectly acceptable and the Tissot PRX is the next best thing after sliced bread
2. two tone watches are not tacky at all
3. you need to have your name down on waiting lists at a minimum of 3 Rolex ADs before you have any credibility in this game
4. owning an FP Journe is your one grail
5. amendment : owning a grand complication Patek Philippe, that you have not seen in three years because it is stored in an underground bank safe two hours away from your home, is your grail
6. no collection is complete without at least 3 G-Shocks
7. Grand Seiko is better than Rolex in every aspect (but you still badly want a Batman and a Yacht-Master)
8. your watch pics shall be shot with a Leica camera 
9. It is fine if your €5000 watch runs fast 20 seconds per day. Being obsessed with accuracy is not cool
10. any watch looks better on an Erika's original strap
11. Furlan Marri is almost as good as Patek Philippe
12. and, finally, Hodinkee have lost their touch since LVMH bought them

be guided accordingly, and you're guaranteed a minimum of 5000 followers on IG

I could not have put it better.

and has a nice pop of colour on the dial

My advice for watch collecting is short. Two words in fact: DO NOT!

1. Buy the watch (within limits). Staring at pictures/videos definitely do not tell the full story. It doesn't hurt you too (unless you have nickel allergy, in which case go titanium).
2. Don't buy the watch. After all, who needs watches? They're also a pain to sell if you buy the wrong one. One or two nice watches are all you need after all.

This ^

"You get a lot of watch for the money" - $6,000 watch

Don’t get something just because you don’t have a thing like that. Just buy things you REALLY like. 

Life is short and you're going to die! So YES! sell the car and buy that watch!! 😂

Don't buy a watch if doesn't  work and believe the person who selling you the watch does work. And you buy it anyways and find out after putting a battery the watch does't work and you go back to the person who sold it to and he's no where to be found!

Surprised no one has mentioned this one: 

Buy the seller, not the watch.

1- no rush, do your homework before purchase.

2- buy for yourself, not for others.

3- buy a watch to wear the watch, so expect some scratches on your timepiece.

4- don't buy a watch if you don't have the same amount saved to replace it right away.

5- mechanical watches will need a service eventually due to friction within the movement. Consider the cost.

6- learn from your mistakes (buying/selling)

7- do your best to try the watch on your wrist before.

8- you don't need that many watches.

9- don't buy the hype.

10- if your watch is expensive, don't tell your partner. No, don't.