In-Game Trades

In Diamond and Pearl there are some trainers throughout the game who are willing to trade you their own Pokémon for one of yours.
Heartless Fools! They want to trade their Pokémon that they loved and nurtured away for someone else’s... tsk tsk. That's not the way of a Trainer. xD

Anyway, there are 3 people who trade with you before defeating the elite four:

Trade #1

A Woman in Oreburgh City wants to trade away her Abra for a Machop. Abra can be hard to catch if you don't have the right equipment. So, if you are having some trouble, go catch a Machop on Route 207 and trade with her. Her Abra is holding an Oran Berry.

Trade #2

A little boy in Eterna City wants a Buizel. He is willing to give up his Chatot for one (He's Crazy! xD). Go catch a Buizel on Route 205 if you want this bird. The Chatot is holding a Leppa Berry.

Trade #3

A Woman in Snowpoint City wants to trade off her Haunter. She wants a Medicham in return. Evolve a Meditite or catch a Medicham on Route 217 or Victory Road if you want to trade. Gastly is very common in the Old Chateau... but if you don't want to take the time to train one, go for this deal. The Haunter is holding an Everstone (The reason why it doesn't evolve when you trade it -_-)