1- get data however possible. Make a deal with your parents.
2- focus on 1 or 2 specific items/brands of stuff you and your friends like initially and you have some knowledge of like shoes with good bottoms and no cracking.
3- don't waste a Penny of your time money on a shirt if it doesn't have an actual tag, was made in China, or on a gildan.
4- Don't spend one second in the kids section. Kids outgrow clothing so most parents are cheap. Rich kids buy new clothing. It's a flooded category.
Rules of thumb:
The clothes that are a little more expensive ($10+) will likely still be there tomrw. Take pics of them and go home and look up the resell value. Go back next day and buy the best stuff that is still there. Maybe even hide that shit or put it in the kids section so that most adults won't see it. Most huge profit items will cost $10+. Majority of non-reseller customers are thrift cheap and don't touch the stuff over $5.
Clothing wirh utility is valuable. Staying dry is crucial, especially for people who work outside or work outdoors in freezing cold. Yeah you can get some waitress shoes at Walmart for $5 but a pair of steel toe water proof boots for someone who works outdoors isn't cheap. All north face type jackets aren't the same but not all thrift stores know this. Don't buy the cheap basic fleece for $10, look for the hooded jackets wirh hoods and liners and that are designed to keep outdoorsmen (or skiers) in Colorado from freezing to death but are priced like the basic ones.
Several brand blank white t shirts are surprisingly money. Guys are really particular about how they wear, especially suit and tie guy who needs something soft, that won't bunch up, or irritate their skin all day, and isn't trying to have wet arm pits showing at work. They have their favorite ones and brands foolishly stop making them or make small changes that make that old version rare. Brand new or never worn are very common to find, very often vintage. and what you're look it. People get a 3 pack for Xmas and if it's not their fit/model it sits in a closet for 5-10 years. It winds up at goodwill as dead stock.