Although there are other internet auction sites, eBay is a popular site and using them as an example is not in any way an indication that they are at fault or are anything other than a reputable auction site.
According to eBay, most of the disputes are merely misunderstandings and can be resolved through buyer and seller communication. If not, eBay offer a dispute resolution service, more details of this service can be found on their website.
The Citizens Advice have a dedicated Consumer Advice Helpline (0808 223 1133) which you can contact, if something has gone wrong with a purchase. Useful information can also be found on their website, please see the links in Related Information for further details.
If a dispute resolution service does not resolve the matter and you believe you have been the victim of a crime rather than a misunderstanding, you can make a report to your local police either via their 101 or via their online reporting form. Please see the link in Related Information for force contact details. If you or the other party are overseas you will need to make the initial report to your local police force who will then make enquiries with any other force if appropriate.
The police service cannot and will not check the buyers or sellers details prior to you making the purchase/sale. If you are in any doubt at all about the buyer/sellers authenticity then do not go ahead with the purchase/sale, see question in related information for tips on buying goods from the internet.