If it was me, as a private seller:
Cross check the values by looking at live items and 90 day solds on eBay. Use Google lens to image search if needed and get an overview of what you have.
Very low value - charities will take them and worst case they will end up going to a rag mill where the charity will still earn a bit of money.
List anything known to be worth under £10 on Vinted. Buyer pays the postage, it is cheaper than on eBay. If it doesn't sell on Vinted and has no interest move it to eBay after a month or so.
Anything worth over £10 with a value you are sure about, list as a buy it now on eBay. Anything unknown, auction it on eBay.
Mailing bags are cheap if bought in bulk. I think mine are around £11 for 200, I can get them cheaper but find a happy medium with bags that are well made and 100% recyclable. Clothing is so easy to pack and post compared to what you sell now.
Listing can be easy if you want it to be. As a private seller, don't be concerned with measurements too much, it is ok to just picture well and list it rather than not list at all. If a buyer wants measurements they can ask. Just make sure you check items thoroughly for flaws, if they could have shrunk or be a style where the tag size doesn't fit the style today then mention it. The garden can be a good place to picture clothes in natural lighting.
Don't be afraid to use a courier like Evri to send clothing. They are unlikely to damage it, a small parcel is £2.94 with Packlink on eBay and the saving vs Royal Mail adds up if you have a lot of clothing to ship out.
If you are selling t-shirts for example, you can always save money by sending as a Royal Mail large letter but the packaging costs will be a bit higher as you need rigid large letter style packaging.
