I wish I could give an easy answer to this question but the truth is the answer will be different for most people. Every city, county and state has different resources locally. Things people sell at garage sales, thrift stores and flea markets near us here on the Gulf Coast would be markedly different for someone living in other parts of the country.
For instance you will find no Anthropology clothing labels here nor will you find Dansko shoes here however if we travel slightly west to New Orleans we can find those things because they have outlets there. Ok those may not be the best examples but my point is the availability of things vary based on whats being sold in your area.
You have to learn to look for things that most people would ignore. FOR EXAMPLE TONY? lol, ok yeah I can nearly hear you guys questioning this. Like I say things vary location to location but for the sake of argument I will give you a very real example ...
Remember this guy? yep its a " My Pet Monster " plush doll from the late 80's like 1986, 1987, somewhere in that era. Anyway it sells for around $400.00 on ebay. See how calmly I said that, lol. Most people would have seen this and said oh wow another dust catcher but because you had the knowledge of the value of this thing you said WOW!!! It's no different with any other category your interested in.
The more things you sell the more your going to learn about what sells and what doesn't sell. Find something your passionate about, do research on that type of stuff and see whats valuable and whats not. Try to get your items cheap and do alot of experimenting and before long you will find many things that do well and those are the things you will always be actively looking for along with many new experiments every week.
I hope this helps