Taking pictures of your stuff to resell online is a super basic part of the process. What you will want to keep in mind for any item to resell are these key things:
The key to taking quality pictures across the board, whether it's clothes or any sort of hard good is good lighting and a clean background.
Good lighting doesn't always mean need any sort of lighting equipment. If you have great natural light spilling into your house - use that! Natural light is best!
Some people use day lights or clear light bulbs in lamps and whatnot that they already have. Some people buy ring lights. Some people buy lighting kits. The choice is up to you, of course, but you will definitely need to make an effort to take good pictures.
Ring lights are all the rage nowadays. And for good reason, if you watch an interview on TV, you might be able to see the reflection of ring lights in the person's eyes. It's a weird small detail, but it's what I notice - the small things.
There are light kits and light boxes to fit in anyone's budget, from $13 to over $140. It's not mandatory or necessary to purchase any sort of lighting kit, but if you're lacking good light in your house or space, then I would definitely consider.
And if you're handy or savvy enough, you can DIY your own lighting system!
This one is a complete and total personal choice. Many people have asked on social media what kind of background is preferred - crisp, clean white, a white furry rug, a white sheet, fake wood, white wall, neutral wall, real hardwood, a door...any and all of the above are choices of many a seller. I started with a really crappy set up. I used a few yards of fabric from Joann (gray and white chevron pattern) with a hanger and that was it. Somehow I made some sales, but I have since upgraded. In fact, I'm looking to upgrade again. Some buyers say that seeing white background after white background gets redundant and boring. However, I think a white background is a such a clean look and is free of any sort of distraction. Currently, my background is fake wood sheets of thick paper I got from Michaels. But I need large surface area, so it's all taped together and looks a little wacky IMO.
Pro-tip: Google loves a white background.
I'm going to check out Lowes or Home Depot for some wood paneling or look into one seller's suggestion of rollable PVC material ? Sounds interesting and easy.
My biggest suggestion or tip would be to NOT use a highly or brightly colored pattern of any kind. No crazy bedspreads, towels, blankets, and I saw some kind of glittery reflective background once. NO. That's terribly distracting. If you're selling clothes, I would also suggest keeping them off your hardwood floors, though they may be pretty, it's still a floor and some people might be turned off to see their potential new piece of clothing on your floor.
Another suggestion about backgrounds is make sure your actual background is not messy if you're taking pictures without a set up. Meaning, your bedroom, living room, basement or wherever is not clearly cluttered or messy in the background. That's a total turn off to buyers.
You could also try thinking outside the box - there is one seller who tosses his clothes for sale on bushes, logs, trees and he actually sells stuff regularly! Another seller I used to be in a sharing loop with would use her backyard as her background. In either scenario, they're getting natural sunlight so you can't beat that! I'm tempted to use the natural beauty of my backyard, but haven't quite gotten the set up right, so for now I use my taped up fake wood background.
A note about using sheets or other fabrics - any wrinkles or folds WILL SHOW UP IN THE PICTURE. Whether or not this bothers you is up to you.
For inspiration for different ways of taking pictures and background ideas, you should definitely spend some time on Depop. There is nothing standard on that app! If it suits you, this will help you stand out on Poshmark! I can't say that it'll translate well to Mercari or Ebay, but you never know. Each selling app has it's own ambiance, look, and feel. Sometimes standing out is the way to go and sometimes just the basics will do just fine.