It will take much more time to sell stamps individually, but when it's all said and done you'll make a lot more money that way. Time yields money. Selling everything as one lot will go quickly and be simple and easy, but you'll make far less that way. So it depends on what you want.
I'd sell each item that's worth more than a few dollars separately (if I had the time!). Some could perhaps be sold as groups of stamps from the same set, if you want to simplify things a little.
Add a reasonable and modest shipping charge to each sale, but if a buyer purchases more than one item, you might lower the shipping price for each subsequent item. High shipping prices scare me away fast, so be moderate about them.
Either give no description at all and just say "as is" or "as pictured," or be precise in your description -- used, mint lightly hinged, mint heavily hinged, and so on. For stamps worth more than maybe a few dollars, I'd try to include a picture of the back of the stamp, too. More work, though. If I see "no returns accepted" I wonder what the seller might be hiding, but you can do that if you wish. I imagine most buyers don't even bother to look. Still, accepting returns if a stamp is not what it appears to be does seem fairer. Up to you.
Either list at a purely "buy it now" price or as an auction with bidding -- or offer both. I prefer "buy it now" most of the time since it simplifies and speeds up the process. But that's just me, and I've gotten some good bargains a few times by bidding.
As for what price you ask,I'd sell at a small fraction of catalogue prices since higher prices generally go unsold. That's your choice, but well below half catalogue value seems best unless a stamp is genuinely very hard to find. About 1/3 of catalogue value seems about right to me, give or take, and if I find stamps I need that are in good condition at much lower than that, I pretty much can't resist.
To lighten the burden a little, you might go slowly and offer only a fraction of your stamps at any one time. It makes the process easier and more manageable. But it will take much longer, of course if that's a concern.
Oh, one more thing. When a seller lists by including a Scott catalogue number they are far more likely to be seen by buyers in the U.S. and Canada since most buyers will search by catalogue number. Listing with only the Stanley Gibbons number means most U.S. buyers who search by number won't even see the listing. If you don't have a U.S. Scott catalogue, buy a used copy on eBay
cheaply so you can include Scott numbers. Little things like this do matter.