The following are items in G/S/C which can be sold for high prices, obtained mostly through traded Pokémon from the first generation or wild Pokémon holding the itme:
Big Mushroom
Tiny Mushroom
Big Pearl
Pearl
Gold Leaf
Silver Leaf
Stardust
Star Piece
Brick Piece
Other ietms you can sell include the famous Nugget and the not-so-famous Slowpoketail.
These can all be sold for high prices, varying on what item it is. Also, keep in mind that obviously with Big Mushroom will sell better than the Tiny Mushroom, the Gold Leaf is worth more than the Silver Leaf and so on.
~Psychic