Probably a matter of the kind of plants you are looking for.

When are these sales?

I have been to the master gardener sale up in Snohomish county several years ago. I got several nice plants for really really cheap - a redtwig dogwood, perrenial geranium and hellebores - they are still thriving!

I have never been to a swap.

The sales are all Sat. the 6th I'll post the links I am using.
http://forums.gardenweb.com/forums/load/expacnw/msg0421294329506.html
http://www.bellevuebotanical.org/events/fmevents_current.htm
http://www.metrokc.gov/WSU-CE/Gardening/GardenResources/MGPlantSale.htm

I'll be going to the swap. The way it works is you bring a pot-luck dish and whatever plants you have extras of. You can either arrange swaps before hand for particular items, or you can just arrive with yours, and take away whatever you like from everyone else's extras.

You get to see someone else's garden, chat with people and have some good food. There will be another in Bonney Lake next Saturday. The one this weekend is in Shoreline.

Just came back from Mid-Atlantic Gardeners plant swap in Burtonsville MD. (near Wash DC) What a great time. I took tomator,pepper and eggplants, blackberry lily and daylilies. Big catch were two Brugmansia's. Can't wait till next year. Steve in Baltimore.