Again, how do you cost your “time”? I asked upthread but no one answered. Is it an hourly rate at NMW? Or do you just bump the postage up by a couple of quid?
Bluntly, you don't! They aren't employing you, just buying an item from you, so your time isn't worth anything to the buyer. If you're a private seller, getting rid of old stuff, it's effectively a glorified car boot sale - nobody would dream of charging for their time to sell there; if you're a company, you probably sell in volume, thus making the total rewards more worth your time.
As a PP said, surely you would multi-task when going to the post office and take several items to post, do some shopping or whatever.
Effectively, your own time is worthless for these purposes. It's up to you whether the financial gain from selling something is worth your while doing it, also considering the freed-up space in your house and getting any money back from an unwanted item; but either way, your item is worth what it is worth to the buyer and that's it: not your time in listing, preparing or getting it to them.
Unless you're a big-volume business seller and/or it's your only income, it's more realistic to see selling on eBay as a hobby with the side benefits of clearing a bit of space and bringing in a bit of money.