Something I've learned over my nearly 12 years selling online, go with your gut! When I am out and about shopping I don't even have one of those new fangled phones to check if something is going to be a good seller or not, and yes often I get some duds BUT many times I hit a good one meerly by listening to that little voice in my head that says.... hmmmmm that looks interesting.
Someone once told me if you are not losing money on 10% of what you buy for resale you aren't buying aggressively enough to hit the big one! And this has worked well for me over the years. now you need to stick with something you know at least a little about, enough to be dangerous so they say. For me I tend to veer towards glass and pottery, mostly because I'm fascinated by all the different styles, types and forms.
Most glass is unmarked and you just have to get a sense of what is good glass and what isn't, best way is to handle as many pieces as you can! Go to antique shops and other places where you can handle quality glass that is already identified, soak it in... then when you are out shopping for resale pieces you can start getting a feel for what might be a good piece.
Same goes with pottery, good pottery just has a feel about it. I'll give you my latest example and success story. I picked up this interesting wall pocket at a local thrift store, why? It was different, and not knowing a thing about it I knew for the price I could even sell it as a contemporary decorator piece so home it came with me. I then deciphered the name on the back, Richenda Stevick 1935. Hmmmm.... wasn't much on a google search except for some information about a piece someone was selling back in 2002! They called it CA Faience, so I did a bit more research and came up with a good enough description to list it on Ebay. The fun part is Ebay was having a listing sale so I started it at $79.95 why not? I immediately got the opening bid!! Wow! I would have been pleasantly surprised right then! It sat with a few watchers all week and ended at $191.34!! Can we say I did a little happy dance?
So moral of the story... take a chance if something speaks to you! Is it unusual? Weird? Seems to be one of a kind? Go for it if the price is right (my rule of thumb under $10 for the unknowns). Now go find that treasure!