SELLING TIPS:
Put aside an hour to sell/upload (Sundays are good for listings, as statistically it’s when most people are casually browsing on the sofa - or if using eBay, you can schedule your listing on eBay to finish on a Sunday) and upload a few items at once. Natural daylight is often better for photos and showing flaws.
Consider seasonality. It pays to think about what people are thinking about - coats sell better in Autumn/winter than they do in summer, for instance.
You'd be surprised what sells. I once sold an old (working) Gameboy from the 90s for £25 to a collector on eBay. It was old, not the cleanest and didn’t have any use to us any more - win win!
Obviously, it goes without saying all of this should be declared as extra income to HMRC (there is, however, an individual tax free ‘trading allowance’ of £1,000).
Vinted.co.uk
GOOD FOR: High street fashion.
You can sell designer as well but it definitely seems to excell at selling lower priced High Street pieces - you can sell bundles and combine shipping if someone buys more than one item, too. The best bit? No listing or selling fees.
GOOD FOR: Designer fashion
It has been a while since I’ve sold anything here, but the good thing about designer is that if it’s a popular brand, it will hold its price. Vestiaire is used by the fashion community, so you’re more likely to sell less well known names (ie, brands I love like Manu Atelier) than you are at a car boot or eBay. BUT they do take around 14% commission.
eBay.co.uk
GOOD FOR: Items in good to excellent condition.
It's great for selling clothes that don't fit/you no longer wear but mainly when there isn’t too much wear and tear to them (I personally find that eBay buyers are a bit more pernickety about the condition).
GOOD FOR: Furniture
We sell a lot on Gumtree, often old furniture and items that need to be collected. Chris sold some camera lenses too recently, but it’s important to remember to meet someone in daylight and not at your home address, especially when you’re selling more expensive items
CeX (UK.webuy.com)
GOOD FOR: Electricals and DVDs.
I sold my Apple Mac laptop here (make sure you wipe it before taking in!), also good for selling old iPhones. Fun fact: we also watched the entire series of Game Of Thrones by buying series one on DVD, then kept exchanging it for the next series until we’d caught up – cheaper than a Sky subscription!
Car boot sales
GOOD FOR: Everything else.
All your bric a brac, lonely mugs, that photo frame you accidentally ordered in the wrong size, and the shoes you think are in too bad a state to sell. Book a date in advance, so you have a focus, otherwise you’ll have hordes of stuff just waiting indefinitely (and trust me, once you start collecting, you might turn a bit Marie Kondo and the sell pile will just keep growing). Check out Carbootsales.org to find a sale near you. 
