Sergeant Curtius - Found at the Cyseal Harbor Warehouse.

Snorri - Completing 'The Strongman' mission will remove your ability to trade.

Entering Luculla Forest may also cause her to disappear. Progressing the 'A Mysterious Murder' quest past questioning Roberts will cause her to disappear.

Evelyn - Found in the healing house in Cyseal.

Merchants that sell legendary items include: Furthermore, they seem to sell items with item level around your party level (+-1 of your party level), always. Evelyn, Loic, Captain Curtius, Hortun had only non-magical items available for purchase at levels 2 and 3, upgraded to selling greens at level 4, and only began selling legendaries at level 5. It also appears that you must be a certain level or have progressed to a certain point in the game in order for these merchants to sell legendaries. If you are a relatively slow player and take your time, you can easily get two restocks per level. Their inventory seems to restock when you gain a level but also every few hours in-game, so you may want to try checking them once when you level up, and once again before leveling up. Reloading the save and initiating the trade again will spawn a new legendary item. If you have plenty of gold or are able to pickpocket them, it is a good idea to save before trading with them. Several merchants throughout the game sell legendary items.

Divinity original sin legendary items