>How do you get used to selling things?
You just do, eventually. Do it once, twice, three times, doesn't have to be often, just do it; eventually you get better at setting up your listings too.
>How do you make a nice sales post?
Take good, clear pictures of the things you're selling in good lighting, including pictures of any damages. Personally I like seeing closeups of nice details but it's not a requirement.
Also, fully disclose any damage or modification the item has. Please be honest about it, it sucks buying an item described as "like new" and finding damages the seller never mentioned.
Write coherent descriptions for each item detailing its condition; I like including a reason for selling the item but, again, not a requirement.
I don't know if you've never sold anything before but in case you haven't, protip: Keep the boxes, plastic bags, and bubble wrap that your new purchases come in and reuse them for shipping the stuff you sell. The former will make shipping more expensive especially if your local post office has ridiculous fees, so feel free to forgo that step, but generally I find that reusing these materials makes shipping things significantly easier.
Also, it should be obvious but apparently needs to be said: Please mention in your listings or terms whether you have pets or if someone in your household smokes, and please clean garments before you send them out; this includes lint rolling them if you have furry pets.
Good luck, anon!