Yeah, I buy a very high percentage of my clothes on eBay (and also Poshmark, but it ends up mostly being eBay).

What I don't buy used:

- Socks - Underwear - Gloves - Knits in general, as they're too likely to be messed up, though with the odd exception for pieces unusual enough that I figure it's likely they were treated OK, provided the price is low enough I can take a gamble. I think all such exceptions have been 100% linen or ~50/50 silk/linen blend sweaters (these are warm-weather sweaters, basically) - Jeans. If they get creases and fades I want them to be from me. Plus I have my size dialed in on Levi's STF 501s and I can already get those for like $40 on sale, so... what's the point? - Modern sportswear in general. I don't have much of this, but what I get, I buy new (though from e.g. Sierra Trading Post, if I can manage it)

Pretty much everything else comes from eBay or poshmark (exception: I don't think quite half my shoes are used, but a lot are).

Belts, ties, trousers, shirts, jackets, coats. Ebay or poshmark.

Shirts: I've got my sizing figured out really precisely with four or five brands. I can shop these really well by size tag. Like, I know with one Japanese brand I can get the "slim" fit of their very-largest Japanese size (these are neck + sleeve measurement shirt sizes) and it'll fit me great for a modern-fit button up shirt, except the sleeves will be a little too short (in the longest sleeve they offer! And I'm not even that big! LOL). I can get the "New York" "slim" from the same place, which they offer with a size one larger than that, and it'll be absolutely perfect, damn near as good as if I'd had a shirt custom made. I know stuff like that about a few brands. They're all nicer brands, so the sizing is quite consistent. All I have to do when I want a shirt is set a few eBay saved searches, and wait for one I want to come up (if there's not one on there already). Sometimes I've even snagged batches of shirts from someone with my size, resulting in stupid-low prices (like, $10/shirt) for things that look like-new.

Jackets: mostly blazers and sport coats. I know my body measurements, and I know the measurements of jackets that fit me well (arm length from shoulder and from pit; waist at middle or top button, depending on 3 or 2 button; length down the back; chest measured across pits; shoulder, front and back measurement, seam to seam). I have a sense of how to size up for winter garments that have thicker fabric and under which I'll probably want to wear thicker clothes. I know the range of standard jacket measurements (e.g. "40R" for a 40" chest, regular length) I'm likely to find what I need in. The vast majority of sellers provide enough relevant measurements that I can achieve an almost-perfect hit rate on these, and the nicer the piece the more likely they are to provide them. I'd say the average I've spent is $100-$120, and some of the ones I've got would have been . I've leaned on these measurements to also get things like a cotton canvas chore jacket, and a leather jacket. Brand knowledge is all but useless for sizing here, jackets vary far too much and many have been tailored. Closest it gets to being useful is that I know a couple outdoors/sporting brands that either make or used to make sport coats, and that theirs run way large (they probably expect that you'll need to move in them, and that you'll wear heavy clothes under them) so not to automatically skip over them because the nominal size would be too small in ~every other brand.

Trousers: Waist measurement is a must of course, nominal trouser sizes are basically gibberish even in good brands, and many trousers have been altered in the waist. Leg length a must (too long is fine, many nicer ones ship intentionally very-long anyway so you can alter them to your need, but too-short is a problem), measured crotch to end of leg so you're not including the rise. Ideally also leg width at the ankle, and rise (crotch to top of waist), though those can be sort-of eyeballed. Many listings will let you know if there's fabric to let the waist out or leg down, and roughly how much. Like with shirts, I have a good

Suits: for a 2-piece, it's just jacket + trouser, there's nothing new here. For vests (if it's a 3-piece, or if buying an odd vest) the main thing to care about is pit-to-pit chest, which I find to be a little more forgiving (I can go very-slightly smaller) than a jacket provided the vest material is on the thinner end, and maybe the length neck-to-hem, especially if you've got a notably long or short torso.

Coats: Like a jacket, but size up an inch or so, maybe more (for some styles that are meant to be worn very loose, a lot more, potentially). These may go over jackets or other thick or layered clothes (e.g. heavy sweaters), and generally you want them to have a looser fit anyway. If you buy them like a jacket you'll find you can only comfortably wear them over a shirt, which makes for a pretty limited coat. Or, if you have measurements of existing coats you like, just base your decisions on those (basically same measurements as a jacket)

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It looks like a lot, all laid out like that, but if you already have clothes that fit well in each category, it's really just a half-hour with a surface to lay them flat on, and a measuring tape. Pro tips: measure several examples of each as there's probably a small range of each measurement that works well, pay attention to material thickness and how they fit over different thicknesses of clothes for e.g. jackets to get a sense of what to look for for different seasons, and check with fit guides online to make sure these clothes really do fit correctly (they may feel OK, but look off in ways that may be hard to pin down if you don't know what to look for), and measuring clothes that don't fit quite right can also be useful to figure out what's plainly too much, or too little, in a given dimension. Also, consider as you try on for fit stuff like "do I prefer these trousers to those because of, say, the rise? OK, so I need to make a note of which rise measurement, specifically, I prefer..."

Boom, you've got what you need, and will only rarely need to re-do any of that (waist and chest, especially, may shift a little, and we all get shorter eventually, but otherwise you're good). Measure yourself, too (true waist, hip, chest, maybe neck... I also have hand [around, at the knuckles] and head for gloves and hats, LOL) and you're solidly ready to buy clothes with reasonable confidence online, used or new. If you do get something that fits wrong, measure whatever part's not fitting right to help refine your criteria.

Measurements of your own body are mostly helpful for buying new. Lots of retailers will provide size charts based on body measurements, not garment measurements. For used stuff, it's gonna be 100% garment measurements, which will always be at least a little larger than the corresponding body measurements (so it's simplest to just measure stuff you have that fits well, for this)