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I want to know which gifts I should hang on to and, by exclusion, which gifts can be sold to merchants without fear of having to re-buy them to appease that one snotty NPC.

So far I've seen the following:

1st act

• Trish: Trish's potion (obvs)


• (Buxom) peasant woman in the village on the outskirts of Vizima:
  Tulips (A corpse in the area will have them). Reward: good time.


• Beggar woman living in the village on the outskirts of Vizima: White Gull, will reward with Eternal Fire signet ring.


• Old woman living in the village on the outskirts of Vizima who doesn't talk to strangers: Unknown, will accept bribe. Rewards with a story.


• Woman in white dress living in a house on the outskirts of Vizima:
  Wild flowers, will give Tulips as reward.


• Old woman living in the house opposite of the chapel: Any food (Bread
  is most cost-effective), will tell a story, possibly giving some
  glossary entries.


• Vesna, after completing her quest, at the mill: any wine. Reward: good time.

Act2

• Jethro, at the dungeon: Fisstech, rewards with information. useless unless part of a quest.


• Gardener at Vizima's temple: any food, will reward with a tale or roses of any color you choose. Repeatble.


• Herbalist at the market of Vizima's Temple district: Red shawl. Reward: information about where she gathers herbs. useless.


• Bootblack, near where you emerge from the sewers in Vizima: Shadow People, or the story of His Majesty's Secret Service. Get the book by outdrinking Thaler. Reward: Prophet Lebioda's signet ring, useful for getting past Shani's landlady.

The best resource for this is the Shopping List over at the Witcher Wikia.

The 'condensed version', totaling up all of the various by chapter lists, provides these aggregate numbers:

• 40 Beer, 1 Wine, 1 Rose Wine, 5 Sodden Mead, & 3 Temerian Rye




• 20 of any food item. In addition, you'll need 5 Bread, 1 Dried Nuts, and 2 Sugardolls




• 1 Daisy, 1 White Roses, 1 Red Roses, 1 Tulips, and 1 of any kind of flowers.




• 1 Lady's Shawl, 2 Gold Shawls, 2 Red Shawls, and 2 Red Gloves.




• 1 Amber, 1 Ruby, 2 Diamonds, 2 Sapphires, and 1 of any precious gem.




• 1 Silver Amber or Silver Ruby ring. Which depends on one of the major choices you'll make in Chapter 3. Additionally, 1 of any kind of ring.




• 5 Beggartick, 5 Celandine, 5 White Myrtle, 6 Fool's Parsley, and 1 each of Feainnewedd, Honeysuckle, Naezan Salts, and a Wine Stone.




• 1 Red Meteorite




• 2 Fisstech




• 1 Perfume, 1 Swallow, 1 White Honey, 1 White Raffard's Decoction, and 2 White Gull.

And finally, for skinnable items, you'll need

• 
  3 Alghoul Marrow, 6 Alp Fangs, 3 Archespore Juice, 11 Barghest Skulls, 3 Basilisk Hide, 10 Bloedzuiger Blood, 6 Bruxa Blood, 6 Cemtaur Jaws, 5 Cockatrice Feathers, 4 Death Dust, 6 Dog Tallow, 10 Drowned Dead Tongues, 3 Drowner Brains, 11 Devourer Teeth, 3 Echinops Roots, 3 Fleder Fangs, 2 Garkain Saliva, 15 Ghoul Blood, 4 Giant Centipede Armor, 5 Gravier Bones, 11 Kikimore Claws, 9, Shadow Dust or Shimmering Dust, 11 Wolf Pelts, 3 Wyvern Eggs, 10 Wyvern Steaks

If it's not listed above, or you've already made use of one, and you don't have a use for it yourself, you can safely sell it. Consult the Shopping List link above for a more detailed chapter by chapter breakdown of which items are useful where and when.

As you said Trish's potion, Tulips, White Gull, Wild flowers and food(bread).

Chapter II

• Prostitutes - in the Temple Quarter will accept any flowers.




• Gossip - in the Temple Quarter during the day near
  the Hospital or the detective's house. Give her Gold Woman's Gloves,
  Red Woman's Gloves, or a Diamond. You'll probably have to talk to her
  several times before you have the opportunity to give her a gift.




• Declan Leuvaarden on the Dike give him 3 Salamander Brooch to prove your loyalty and receive 600 orens.

Chapter III

• Couresan - She's outside the New Narakort Inn. Pay her with orens or a gem (an Amber will do)




• Blue-eyed Lass - She can be found in the House of the Queen of the Night. Give her 500 orens (300 if you show her the House of the Night Signet Ring) or a Sapphire.




• Townclerk - on the second floor of Town Hall in the Trade Quarter will fool around with you if you call her a naughty girl and give her a Diamond.




• Noblewoman dressed all in white in the Trade Quarter near the Town Hall will sleep with you if you give her a Silk Scarf.




• Noblewoman near Triss' House in the Trade Quarter will sleep with you for a Kikimore Claw.




• Celina - Give her any ring and she'll sleep with you




• Peasant Women - You'll find her in the Inn or wandering around the Village. Give her a Sugardoll and she'll sleep with you.




• Elf Woman inside the Elven Cave by the Lakeside will sleep with you if you give her food after completing the Daily Bread quest.

This are just some of the things you can use mostly to get some "affection".The list is not 100% complete so unfortunately it doesn't give you good insight on what gifts to sell immediately.