Collecting in Sims 3: A Master Class
If you have been playing Sims 3 for very long, you have no doubt tried your hand at collecting things. The first advice I have for you is: save up your Lifetime Happiness points for the Collection Helper. I'm sure it's possible to collect things without the Collection Helper, but why would you want to?
The basics of collecting are simple. You go out and find things. You keep them, display them, or sell them. But there are a lot of finer points which it can be helpful to learn.
SPAWNING
When you hear hard-core Sims players talking about "spawn points," they mean those little seed areas where collectibles appear. Spawn points are fixed (and in fact there are ways you can stick them on your lot with cheats). However, not all spawn points spawn equally.
Both common and rare items have fixed spawn points. However, common items spawn almost continuously (about every 6-12 hours by my count) whereas rare items spawn much less often (maybe only once a week).
Furthermore, once a common item spawns, it's there for good. You take a Monarch butterfly spawn point, the Monarch butterflies are always there once they spawn. But rare items may disappear if they don't get collected within a certain time span.
(This is why, when you suddenly have need for a Pink diamond, you can't find one just lying around.)
What this means for you is that if you know where a rare item spawns, you can just keep watching it. Check it out once or twice a Sim-day and wait for the item to spawn, then run out and snatch it up. Be sure to go get it right away, or it may disappear!
Spawn points often vary a little from game to game. But if you're looking for something particular, just Google "sims 3 pink diamond spawn" (or whatever). You'll find a lot of forum comments with people mentioning where they found that particular item. It's reasonably likely that you'll find that item in the same place in your game, as well.
SIMOLEONS
Collecting things turns out not to be the greatest way to make money. Cut gems and smelted metals are worth some decent bank, but you have to invest time to collect them, and money to have them sent off to be cut.
The only decent way to make money by collecting is to get to Gardening skill level 7 or better, then go hunting Unknown Special seeds. Plant every Unknown Special seed, and about 1/3rd of them will turn out to be Money Trees. Mega cash back!
DISPLAY
Personally I find displaying collections to be a little disappointing. I mean, they're okay, but I just feel like they ought to be more impressive.
But by far the worst item to display is fish you have caught. If you don't feed them and clean their bowls, they die at a shockingly high rate.
Instead of keeping fish in bowls (unless that's your Sim's Lifetime Wish), collect 10 of that fish, then create a pond on your lot. Click on the pond and choose "Stock Fish" to drop your collected fish straight into the pond. Then you can pull them out whenever you like!