I use Poshmark and eBay for a lot of business casual stuff. More on reddit.com

First measure yourself because vintage clothes are not the same as they are now. Try to figure out the minimum size for your shoulders, chest, waist so that you can easily tell if what you're looking at will fit you because vintage sellers online add measurements of clothing (usually lying flat.. so double it) Maybe if you need ideas, you can look through pinterest or find inspiration from a photo or someone else's collection. You need to have an idea of what you want before you start. Etsy, Ebay, local thrift stores have used items. But to start I'd go to a local thrift and try on clothes that are marked vintage and see if anything inspires you. Like this is the best way to compare your size to the sizes of vintage clothes and figure out what flatters you or if you want to bother in the first place. It takes time but it's an interesting hobby and can be really fun good luck! More on reddit.com

At the moment for menswear it's probably Depop.

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do you ever feel like you're a bad person for making a profit on things that other avid thrifters could be enjoying? do you ever see other thrifters glare at you when its obvious you're buying things that aren't your size? . how many college town menswear-store-specific Gant shirts have you picked up? . how often do you find shirts with custom personal names stitched inside them? . whats the coolest true deadstock you've come across? More on reddit.com