Some individuals sell there collections online via private sale. Purchases are often made through PayPal or eBay. These sellers often share their business on social media groups. If you are looking for groups like this simply search “buy cardname UK”. You will be able to search collections but the good ones move fast! Always use caution and protect your purchase where possible.
Ebay and PayPal offer schemes to protect the buyer. Be sure to pay goods and services on PayPal. These are by no means flawless guarantees so do your due diligence. If there’s any red flags don’t be shy to walk away. Some sellers may wish to meet in person. Do this at your own discretion and meet somewhere public. Often cardshops welcome people trading at their shop.