I started out selling bits and pieces from around the house to raise a bit of money, after DC 1 and 2, XH left and I sold anything not bolted down to pay for basics. I remarried and carried on ebaying. When my DD (DC3)was born she had lots of pushchairs over three years. I'd buy, repair and tidy up, clean really thoroughly or replace covers use for a while and sell on.
I then sold some bits of jewellery and found that higher value stuff resulted in no more hassle than the sub £10 that I'd mainly been selling. A couple of years ago I started running an eBay based online jewellery business.
Now VAT registered and expanded to several other platforms.
My tips are be honest and work out if it's worth your time to sell items. Not every pound is equal.
If you've time to kill and £5 cash in your pocket would make a difference, you can get to the post office or be in for a courier collection then go for it. If you're getting various benefits selling your second hand and unwanted items isnt income so doesnt effect these so £5 is £5.
If £5 would be nice but you'd have to taxi to the post office it includes eBay, paypal and postage fees and you really don't have the time because you're already flat out with life, then work out if after the actual and emotional cost that £5 is really £5 profit.
Sometimes it's better to bundle and batch stuff so you sell bigger lots (three dresses as one bundle but only one postage overhead rather than three separately listed ones and three lots of postage) have all auctions finishing on say a Friday and pack over the weekend to post out/ get collected on the Monday. Most people should have paid by then. Doesn't work as well for buy it now because that can go at any stage.
EBay like tracked postage services and don't acknowledge signed for ones. So whilst a signature on receipt means a buyer is less likely to try and scam you, if they do try, eBay will most likely accept their claim of non receipt as there is no automatic tracking.
It depends on the category you're selling in but many buyers expect fast free delivery now and so you need to factor accurate costs into your starting price or buy it now price. On your sale you have your ebay final value and paypal transaction fees.
It's worth looking out for free listing and fixed or low final value fee offers for higher value items. If you have a £100 pram to sell and can get a £1 final value fee listing thats a good few extra pounds in your pocket.
To price things search broadly what you think the item is and click on sold listings from the menu on the left hand side on the desktop version. You can filter by highest price plus postage and work out where your product would likely fit in. If five similar have sold I'd discount the top and the bottom price and average the three left as a guide.
I make a living at it, so think it's worth it. There are scammers but they are rare and you can follow tips left in this thread to help minimise the risks.