you should check out Listia (
http://www.listia.com) it's like eBay mixed with freecycle and has a credit system (you can buy credits or earn them listing stuff for auction). You're not going to find all the hydro stuff you want but if you keep your eye out I've found plant foods and usefull light fixtures and tools/materials for grow rooms on there. Keep in mind the site is for random/general stuff/merchandise so it's not targeted towards gardening but they do have a home and garden section that you should keep an eye on, you never know what someone else won't need anymore and it's all free on there.
NOTE: This is not a plug for listia. I do not work for listia and as a show of good faith I'm not including my referal link so I don't even get the bonus credits for anyone's sign up. It's just a really cool site to keep in mind when "Hobbying" on a budget.
Peace and good luck to you!