The best purchasable item for transferring souls via trade is Homeward Bones.
They can be purchased for 500 each and sold for 100, and most importantly, have a Held limit of 99. There are many items with a 5:1 ratio, but most have low limits, so only a paltry amount can be handed over on each summon session.
For the price of 49,500 souls, you can transfer 9,900 souls in a single transaction via 99 Homeward Bones.
Blooming Purple Moss Clumps are equally as good as Homeward Bones (5:1 ratio, 99 Held). However, they are not available for unlimited purchase from the Shrine Handmaid until given the Excrement-covered Ashes. They are also dropped by the little crabs in the Catacombs of Carthus, so you can save some money by farming from them.
The second best item is Purging Stones (when bought from the Shrine Handmaid after giving Yuria's Ashes). These are bought for 3,000 souls and sold for 500 souls, making for a 6:1 ratio. This means they are a worse investment than Homeward Bones, but they too have a 99 Held limit and sell for 5x more than Homeward Bones. Large amounts of souls can be transferred faster this way, but you will lose more souls from the worse ratio.
For the price of 297,000, you can transfer 49,500 souls in a single transaction via 99 Purging Stones.