Moggs, first let me say I love that you're offering natives.
I might be your one wish-listed. I just wish more people wanted them. Sometimes if I'm collecting seeds myself, I can get plenty to offer but sometimes I'll only collect a small amount of seeds. (My husband will only wait so long before he's out there with his machine cutting away!) And if it's something I bought for myself and I'm sharing, there's probably a small number of seeds I can spare.
But, there are no guarantees that any or all of your seed offerings will be taken by the end of the swap. What Diana said is true for me. If it's something I really want and I see that there's only one packet being offered, I'll definitely try to grab it up the first day, especially if I see that 12 other people want it too! But with 10 packs being offered, I'll probably feel confident that I can wait to pick one up before the swap ends.
Let us know if you have more questions.