Good on you for asking.
https://airsoft-forums.uk/classifieds/ and
read the banner. You may need to make a few more posts to get access to the classifieds.
If you want to actually
sell stuff, then 60-70% of the current lowest retail value is a realistic price to achieve. Extras and uPgRaDeS don't add much, they're more of a deal sweetener. Used two tones are less desirable than RIFs.
Do yourself a favour and figure out any extra fees and postage up front and list an all-in price, rather than starting to tack on more costs later. Most sensible buyers will only pay by PayPal goods-and-services (not friends and family), or face-to-face. It's quick and easy to send a PayPal invoice / request a payment, listing exactly what you're offering and what you want them to pay.
Understand that postage is at your risk, not the buyers, and figure out what level of risk and cover you want to accept. That's entirely your concern.
Best of luck.