I do a good majority of my picking at thrift stores, so I will echo the "make friends" sentiment. For a while I was going to a thrift store that was pretty high end, and I befriended a girl who ended up showing me the "not good enough" bin in their donation sorting area that was destined for the trash. I got to pick it for free, and let me tell you they got rid of some good stuff! Also do some research (usually just word of mouth, the best ones don't exist on the internet) and try to find tiny local thrift stores and church sales, they tend to yield some of the best deals. Plus, a lot of them are genuinely charitable organizations that are doing real good, so you can feel good twice; once when you get a big smile and thank you when you drop $100 at a church sale, and again when you make $1000 on the items you found.
Honestly one of the biggest sources of hidden gems at thrift stores that many pickers pass over is the clothing. It can be a pain in the neck and very time consuming, but it is super easy to ship clothing (it's not going to break in transit!), and pretty easy to find $1-$10 items to turn into $20, $30, $50, $100, $200 over and over. Use ebay sold listings to find stuff that sells, I usually just type in a general search like "men's Jacket", then set the filter to "pre-owned" and $29.99+ sold price, then scroll and learn. I personally recommend men's and unisex clothing & accessories, the women's clothing market is too complex for me, but there can be huge gains to be made if you put in the time to learn it.
Many people are wary of clothing because of returns, but honestly if you only buy high quality items and take careful measurement, it really isn't a problem. I've been selling mostly clothes for 2 years now, and I've only had 2 returns, and I quickly re-sold the items as soon as I got them back. Even if someone wants to return something, you shouldn't be buying clothes that you can't easily sell again. Also on that note, while you're new to the game avoid gambling. It is easy to try and justify a cheap purchase, but as a person who as probably spent hundreds on total duds I would suggest writing down the brand name, looking it up at home, and then visiting the next day or chalking it up as a lesson learned instead of buying on a whim.
Hope this helps, let us know how it goes!