Acronyms that are relevant for doll collecting and purchasing:
NRFB – Never Removed From Box (if a box has even been opened, it can no longer be considered NRFB)

MIB – Mint In Box
MIP – Mint In Package
MOC – Mint On Card (referring to accessories and outfits)
"Mint" means just as it was when it came from the factory, but contrary to NRFB the packaging has been opened, and perhaps the doll was taken out to put on display.

BIN - Buy It Now (a fixed price, contrary to a bidding auction)
OOAK - One Of A Kind (customized)
FS - For sale
WTB - Want to buy
WTT - Want to trade
WTS - Want to split (outfit/doll)

Jun-Doll.com - Jun Planning's website for all doll types
Pullip.net - official Pullip site
Re-Ment International - food and other miniature props
Megahouse - more miniatures

Shops that I can recommend:
Pullipstyle
The Valley of the Dolls

ShopaDollic - hub for second-hand buying, selling and trading of all doll types
Dolly Market - hub for second-hand buying, selling and trading, mostly Pullips and her family

JunkySpot - awesome for obitsus, US based
Mimi Collection - obitsus, volks EB beauty, eyes, shoes etc.

Sunny's Wonderful World - fur wigs
StarXcrossed/lovehaze - fur and mohair wigs
tinybear/Sleeping Elf - Tibetan wool wigs
JPop Dolls (eBay) - amazingly soft "real hair" wigs, own brand and Monique Gold Collection
Cotin Doll - funky synthetic wigs

Japan Toys Collection Online Store - great re-ment seller on eBay!
Minibarbie Botique - awesome and special action figure/Barbie outfits
Wonderful World of Dolls - Barbie outfits
Peppermint Patty's Barbie Boutique - Barbie outfits
Dodi's Shop Of Stuff - More Barbie outfits, shoes
small-comfort.com - handmade upholstered furniture

Shops that I haven't tried yet:
Blythism - gorgeous handmade outfits (note: has several negative remarks for items not received)
VictorianWoodshop.com - handmade, solid wood furniture
Crafty Jake - beautiful, classy beds

Pullips can wear Barbie and My Scene clothes, and fit in furniture for those series as well.
Some pattern pages:
Patterns at Pullip.net
PuchiCollective (sewing - for Blythe, but they seem to fit Pullips too)
Sticka till Barbie ("Knit for Barbie")
Japanese sewing patterns

Accessories DIY:
How to repaint Barbie plastic furniture
Make your own props (click Dollhouse Ideas)
Printable wallpapers
Printable re-ment
Printable mini posters etc.

Customization resources:
Pullip bodies comparison
Volks EB Beauty / Volks Neo-EB [Teens] / Obitsu HB comparison
Misc replacement bodies comparison
Obitsus comparison
Bodies comparison, different heights and types
More body comparison
Faceup tutorials and resources

Message boards:
Pullip Fiction - mostly for pic fics
Dollie Mixtures - has sub boards for the most common dolls, even Barbie
Pulliplejset - Scandinavian message board
Pullip Style (store Pullip Style's forum)
Valley Talk (store The Valley of the Dolls' forum)