Some flea market items sell online more readily than others. Knowing the categories that tend to move quickly can help you decide what is worth buying when you are trying to resell for a profit, as opposed to buying for yourself.
Vintage clothing and accessories are a perennial category. Items in good condition from recognizable brands, along with denim, outerwear, and well-kept leather goods, often attract steady interest. Check for stains, odors, and missing buttons before you buy, since condition drives price.
Tools, especially older hand tools and good-quality brands, have a dedicated buyer base. Cast iron cookware is another staple, since many buyers prefer older pieces and are willing to buy ones that need restoration.
Books, records, and media can be profitable when you know the market, but they can also be heavy, low-margin, and slow if you do not. Specific editions, signed copies, and out-of-print titles are worth researching, while common paperbacks rarely are.
Small collectibles such as pins, patches, toys, and advertising items can sell well because they are cheap to ship. Look up what comparable items have actually sold for, not what sellers are asking, and be realistic about shipping costs and fees when you figure your margin.
The general rule is to buy what you understand. Resellers who focus on a few categories learn what sells and what does not, and that knowledge is usually worth more than any single lucky find.