In each Pokemon game I usually make a point of trading with each NPC that offers one.
Where are the traders in Sword/Shield? Which Pokemon do they offer and what do they want in return?
There are plenty of trades to perform in Sword/Shield! Here's the ones I've been able to find, as well as some from other sources
These trades are available in both games:
Bunnelby → Skwovet ♂ (Motostoke Pokemon Center)
Nickname: Cheekers, Level: 10, Nature: Mild, Ability: Cheek Pouch, Held Item: Oran Berry, Dynamax: 1
Meowth (Galar) → Meowth (Kanto) ♂ (Turrfield Gym)
Nickname: Cash, Level: 18, Nature: Timid, Ability: Technician, Held Item: Sitrus Berry, Dynamax: 1
Minccino → Cottonee ♀ (Hulbury Market)
Nickname: Candyfloss, Level: 23, Nature: Modest, Ability: Prankster, Held Item: None, Dynamax: 1
Toxel → Togepi ♀ (Hammerlocke - Outside Vault)
Nickname: Snips, Level: 25, Nature: Timid, Ability: Serene Grace, Held Item: None, Dynamax: 1
Yamask (Galar) → Yamask (Unova) ♂ (Ballonlea Gym)
Nickname: Masky, Level: 36, Nature: Bold, Ability: Mummy, Held Item: None, Dynamax: 2
Obstagoon → Mr. Mime (Kanto) ♂ (Spikemuth - Near battle area)
Nickname: Mymo, Level: 40, Nature: Calm, Ability: Soundproof, Held Item: None, Dynamax: 1
Frosmoth → Duraludon ♂ (Wyndon - 4th house down from the stadium)
Nickname: Linear, Level: 50, Nature: Adamant, Ability: Heavy Metal, Held Item: None, Dynamax: 3
These trades are exclusive to either Sword or Shield as noted below:
(Sword) Maractus → Hatenna ♀ (Stow-on-Side - Top of leftmost building)
Nickname: Fringe, Level: 30, Nature: Quiet, Ability: Anticipation, Held Item: None, Dynamax: 1
(Shield) Maractus → Impidimp ♂ (Stow-on-Side - Top of leftmost building)
Nickname: Peepers, Level: 30, Nature: Random, Ability: Anticipation, Held Item: None, Dynamax: 1
(Sword) Vanillish → Throh ♂ (Circhester - Ice cream stall)
Nickname: Reddos, Level: 37, Nature: Adamant, Ability: Inner Focus, Held Item: None, Dynamax: 2
(Shield) Vanillish → Sawk ♂ (Circhester - Ice cream stall)
Nickname: Bluebop, Level: 37, Nature: Adamant, Ability: Inner Focus, Held Item: None, Dynamax: 2
Sources for the ones I was unable to find originally: