if op is nursing, selling plasma doesn't seem like the smartest thing in the world to do (and it's not that great of money anyway, or it least it wasn't 10 years ago when i did it).
other stuff to sell:
CDs and DVDs - put all your music on your computer and burn copies of movies.
your maternity wardrobe.
if you have a baby carrier you don't really like and don't use often, it could sell for $50 or more (or way less, of course) depending on what it is.
outgrown baby wardrobe, if you think you're done.
an extra set of dishes, if you have a "good" set that you don't love and don't use.
your stash from whatever craft/hobby you no longer do.
baby gear you don't use, like a crib set (bumper, dust ruffle and quilt?), high chair if you normally use a booster seat instead, pack & play, exersaucer, swing . . . not that i'm saying people shouldn't use those things, especially with 3 kids under 4 - just saying if you don't, you could sell them.
you can also cut expenses by having a week (or month) of "camp" where you use minimal electricity, minimal gas, basically spending your days in your back yard (and therefore, perhaps, minimal diapers?).