Okay! Here's some information!
Rarity of stuff: There are some items that can only be received in Japanese/European games, some that are DLC, some that can only be obtained via amiibos, and items that are just plain hard to get. For example, the Sanrio items can only be obtained by scanning in a Sanrio villager amiibo card--those aren't sold in the United States (assuming you're in the US and not Europe), and on Ebay the cards are sold at inflated prices, so that makes the Sanrio items very hard to get, thus making them rare. There are also items such as the 24-hour shop items (or the 7/11 set, as it's frequently referred to as). The 24-hour shop set was released years ago as DLC in Japan, thus making those items rare, too. Also, the items that are hard to get are considered rare, too. Items from Gracie's boutique (which is unlocked after upgrading Timmy and Tommy's shop enough times), items from the Bug Off and Fishing Tourney, and holiday items can be considered rare.
What a MEOW coupon is: MEOW coupons are a type of currency in New Leaf. On your Town Pass Card (it's the little red tab at the top of your bottom screen), there are some initiatives. They're things like, say, talk to your villagers, catch a specific fish, sell some stuff, or put furniture in your house. Once you do those little chores, go to the CAT machine located in front of your town hall, and it will give you some MEOW coupons, which you can use to buy items from Harvey at the campground (which I'll talk about in a minute.)
How to trade villagers: I don't personally do villager trading, so I may not be the best helper on this one, but basically people like to use glitches or time travel (which is setting the game's clock to be a different time), and have their villagers move out. On the day the villager is moving out, their belongings in their house will be boxed up, and people will invite other people in their town to invite the villager to move into their town. Sounds complicated but it's really not. If you have amiibo cards, you can also use those to kick villagers out and replace them with the one on the card. People will sell amiibo villagers too.
What the heck is TBT: TBT (also referred to as BTB) is the currency here on The Bell Tree forums. People generally want to use TBT to purchase in-game items from other members, but can also be used to buy collectibles (little stickers that appear in your sidebar-- if you look at mine you'll see a pear, a cake, and a candy collectible), art (look at the Museum Shop thread and some people will take commissions and draw things for you), and many other things. IGB (in-game bells) are also used when doing item/villager trades, but people typically will want TBT. Also, you can't use IGB to purchase collectibles, and very rarely will people take IGB for art. To transfer TBT, all you have to do is look at someone's sidebar where it says "bells" and tap the bells. A little window will pop up asking you how much you want to send them. Type it in and the person will be notified via Private Message. The site will award you TBT bells for posting, but the easiest method is selling stuff in the Re-Tail forum. The common conversion rate for TBT to IGB is 1 TBT equals 200,000 IGB.
The camping grounds: On your town map on the bottom of your screen, there should be an arrow pointing to one of the sides. Go over there and walk up the slope to find Harvey's camp ground. You can use your MEOW coupons there to purchase items you won't find anywhere else. Also, if you scan in an amiibo, that character's RV will pop up at the campground, and you can buy stuff from their RVs. To unlock amiibo compatibility, you wil have to find Wisp's lamp. Wisp will stop you one random day as you're walking in town and ask you to come find him. Once you do, all you have to do is pick up his lamp, take it to your house, and then do what he says.
I hope this helps! Tell me if there's anything else you need.