For postage, if you have a printer then that is easier, but for most options you can use a QR code and have it printed
(Every now and then someone can have a problem with cabinet drop offs that get mislabelled in bulk when the carrier mixes them up - so if you can use your own printer or use a drop off that can print there when you drop off)
I’m lucky that there is a post office counter at the garage near me, so my Royal Mail and Evri go there by default. The next garage along the road has a Royal Mail locker so that’s my next option when the counter is closed. InPost and others are dotted around near me
On both Vinted and eBay’s “simple postage” there are choices in your account details. If something won’t work from you then untick it, otherwise the buyer has their preferred choice.
Bear in mind that Vinteds Royal Mail does not generate a label, but pays you the postage charged. They charge the lowest price for the size you specify on the listing, so if it is a bit heavier than the minimum then you pay the extra from your sale price
Remember to take into account that things need to be packed, so allow for it to be a bit bigger and heavier than you think - if in doubt then package the item as if you were about to post it
For packaging I reuse all the packing I receive plus cereal boxes etc
My friend has a shop so I get bubble wrap, soft packaging etc from her and also take quite a few of her delivery boxes - they are too big 99% of the time, but I cut them up to fit
My packaging expense is a lot of tape and decent bin bags to wrap the homemade boxes
Ask local independent shops and they are usually happy to give away their delivery packaging (it costs money for commercial waste)
I used to use Vinted for clothing and eBay for everything else. I have scaled down my Vinted
It wouldn’t hurt to list items on both (as long as you remember to take it down from the other service once sold elsewhere) or to try different items on the different services
Don’t forget local Facebook sales groups etc - but that means dealing with people so I can’t be bothered