Can someone tell me a good tip for making money quick in Pokemon Platinum?

Battling the 'Rich Couple' is the standard way to make large sums of money in-game quickly. You will at least need the VS Seeker. To maximize the potential money made, equip your lead with an Amulet Coin (found in the left-hand ruin in Amity Square).

Rich Couple

The Rich Couple consists of a Gentleman and a Socialite, found on Route 212, just west of the Pokémon Mansion's front garden. At maximum level, each give a base money of $12000 (D/P) or $12200 (Platinum). With the Amulet Coin, the money awarded is increased to $24000 (D/P) or $24400 (Platinum). Using the Amulet Coin, you can completely fill your purse within 42 battles.

Even better than just carrying an amulet coin while you do that, since you probably have a high level pokemon carry around that pokemon and a full team of zigzagoons or tedyursas with pickup they find items relatively fast and you can just sell the items they find while doing other postgame stuff. They need to be about lv20 then they pick up pretty good stuff on average. Hope this helps

Amulet coin, and honestly in platinum the only way to make money quick is amulet coin and the elite four.