First, it's a good idea to make sure your traders know that you are trading seedlings and just what size they are. If someone is trading a mature plant, they hope to receive mature plants in return, usually. It's rather disappointing to receive a handful of two-inch seedlings in return for a well rooted hydrangea bush.

If it's ok with the trader to receive the seedlings, you can usually ship them safely when they're 3 or 4 inches tall with a few leaves. I'd leave a little bit of dirt on around the roots to help reduce the stress of the transplant. Wrap the dirt/rootball in a moist paper towel and then enclose the rootball in a little sandwich baggy. You can tie it right at the top of the rootball with a twist tie so that the rootball doesn't dry out during shipping. Do NOT insert the entire plant in the baggy. Anywhere the plant touches the plastic, it will rot in transit.

Toilet paper rolls are great protection for little seedlings. You can nestle them inside, after you've wrapped the roots as mentioned.

Use newspapers or packing peanuts to cushion the plants. I always try to find a box that the plants will fit snugly in with little packing. It helps keep them from jostling around a lot during transit.

And always ship your plants by Priority Mail. Delivery Confirmation (DC) also helps ensure the package reaches its destination. The post office is more conscientious if they know the box is being tracked. And chances are, if you use DC, your box will not get lost.

Good luck!