Most rare/valuable G1 accessories?

[mul-ta]

Dream Beauty barrettes. Not terribly expensive, but some of them come in unique shades that are hard to come by. 7-Tales and Dutch combs.

[Sunshine]

Surprise Twin's Mommy's Babies are super rare!
Sweet scoops charm
Jewelry Babies accessories
Stickers for ponies like Yr 3 Sea Ponies, Mountain boys...

I totally agree with lots of the things people have mentioned, those are just the ones that came to my mind!

[Mermaid]

Here is a list of what I think are some of the hardest to find/most valuable accessories:
Conch shells
Prom shs perfume
Shs mesh ribbons
Taf baby ribbons
Soap from scrub a dub tub
Star sponge from scrub a dub tub
Metallic trim ribbons
Snowball from Megan's place
Baby racers comb
Certain color stack toys
Best friend baby animals (Susie and co)
Baby princess sparkles mesh ribbon
Baby dragons
Little buddies from pocket pal pony wear
Megan's sunglasses from by the sea
Jewelry baby jewelry
Sea side baby floats
Big brother bandanas
Strike up the band baton
Lollipop barrettes
Squirrel clips
Mesh ribbons from dance and prance
Bracelets and rings from secret surprise ponies
Birthday pony's necklace
First tooth glow worms
Paradise estate flowers
Paradise estate silverware
Cookery pony aprons
Sweet scoops hat from satin slipper shop
Scoops charm
Pony make up
Shs lipstick tubes
Soda sipping soda drinks
Orange teddy bear brush from perfume palace
Hair netting from perfume palace
Baby lollipops barrette
The make up applicator to the perm shoppe
Elastics to the baby rattles
The present from party time pony wear
Princess clips
The choker from lights camera action
The pendant from hearts and candy
Dream beauty barrettes
The ribbon from flower darling
The paint brush from pretty as a picture
The polka dot clip from get into the groove
Rock and beats guitar brushes
Star fish barrettes
The pink comb from paradise estates
Hair do ponies mirror combs
Roller skate ponies walk man
Petite star picks
Regular star picks
Birthday twins cake and candle and plate
So soft sleepy pacifiers
Bootsie and tootsies bottles
Pregnant pony's twins
Fancy pants mesh bows
Baby ribs and baby whirly twirls bib
Loving family bowties
Rupunzels iluvu clip
Pony nail polish
Birthday ponies present
Princess wands
Rosettes soap on a rope
Puffy and flat stickers.

Hope this helps some!

[hathorcat]

With accessories there is a bit of a difference between "hard to find" and "expensive".

A lot of the items mentioned so far such as the accessories for the Jewellery, Seaside and Sweetie babies are hard to find but they are also very, very desirable, which increases their price. Then you have the Big Brother accessories, they come up for sale a lot - a lot more than many accessories [such as ribbons for the SHS] - but they can get super pricey because everyone is on the look out for them and other items which are not as desirable but much less common sit lower in price. 

I agree with Party, if you show us pictures we will make sure you know what is rare and a price point for things.

[FarDreamer]

Tux's bow tie?  I've never seen it for sale.