I think maybe a 4 to 1 or 5 to 1 ratio would be the most acceptable. And they would need to be of the same armor slot (ex. hands seals into a hand seal).

If this was the case, I would be more willing to help shout groups who need a BLM or WHM yet have no seals to offer that I want. I could take the unwanted seals and later trade them in for a few seals that I do want.