LOOK IN YOUR OWN HOME
Especially when starting out, your own home is a great resource for goodies to sell on eBay. For instance, I regularly cull my CDs, DVDs, and books and pull aside things that I don't think I'll listen to/watch/read again.
And don't forget to think creatively. See my old blog post called "Never Throw Away an Empty Box!" (http://birdhousebooks.blogspot.com/2007/12/never-throw-away-empty-vintage-box.html) I found some vintage department store gift boxes from long-defunct stores, and they sold great on eBay!
CHURCH AND COMMUNITY SALES
Organized sales at churches, community groups, libraries, etc. are all great! For these, you want to arrive early and be sure to take a cloth bag or box to gather your treasures as you shop. My biggest tip is to always look ON the tables, ABOVE the tables, and UNDER the tables, as you never know what you will find.
I have had uneven luck with thrift stores. They are browsed so heavily in my area by collectors and dealers, and the prices tend to be on the pricey side for resale. But -- I have friends in other areas who swear by great buys at the thrift store, and you may have some wonderful stores in your town.
One word about prices for resale -- I never spend over 10% what I think resale will be. For instance, if I think a vintage children's book will resell for $10, I would never pay over $1 for it. (And I certainly would love to get it for less than that!)
EBAY AND OTHER VINTAGE SITES
Again, not as great a venue for resale ... once you add in the shipping I've found you typically pay more than you should for a resale item, using my 10% rule. But -- you never know, and worth looking!
Ahh! My favorite! One of my favorite things to do is have breakfast with friends and go to dozens of yard sales. I am going to say a couple of things that are contradictory to what you may read elsewhere, but these tips work for me. We never go as "early birds," but go after the "early bird" dealers, so we can be a bit more leisurely and also more thorough. (It is amazing what the early birds pass by, and what gets put out later in the morning!) Also, we do not go as dealers, with business cards, book scanners, etc. We are always friendly and chatty with yard sale sellers, and friendly conversation often yields great goodies that aren't yet put out. For instance, "I love these vintage books! Do you have any more?"
A nice thing about yard sales is that prices are not written in stone. I like to gather all the things I want to purchase together, and then ask "how much for everything?" I usually get better prices that way than when I make a specific offer.
Again, a big reminder to look ON tables, UNDER tables, and ABOVE tables ... and to think outside the box about goodies you see.However you choose to search, I hope you find great goodies ... and I would love to hear about them!