From my swap experience over the last several years it seems there are two basic swap event models.
Some swaps are structured so that you receive at least as many as you send in and that is stated in the initial rules. That expectation is set up front and the host determines the mechanism for achieving that. In those cases the players may guide what they receive to an extent, but don't have control of the outcome.
In a swap like this where players determine what they receive there is usually no expectation of a certain return. People list only what they are prepared to exchange for the opportunity to shop for seeds they know they want. The format of this kind of swap is not for people who measure the Kool-Aid in the glass to make sure no one got a milliliter more than they did. (Sorry, it is early and I was remembering how my mom used to have to keep pouring back and forth to keep my sister happy back in the day.) It is also probably not for people with only a few seeds who want to start trading actively and don't want to forfeit a lot of seeds they might have used to join other swaps. If that's the case, they should not offer more than they can afford. If people send in extra seeds, the host might distribute the bounty across all participants or might pad the envelopes of those whose return pales in comparison to what they sent. The disposition of extra seeds is often stated up front, too.
I like both kinds of swaps. Spelling out the rules and expectations at the beginning is the best way to avoid confusion and displeasure from participants, especially among new swappers.
