You ask 'Why won't anyone follow it up?'. The simple answer is that you don't seem to have asked the right people. There are plenty of crimes which the police don't routinely handle themselves. They'll only get involved if they're brought in by another agency. (e.g. the police don't normally handle tax evasion. They leave that to HMRC. Similarly, drug-smuggling rings are often investigated solely by HMRC, with the police only being brought in late on).
In your case, the people who WILL be interested in your plight are your local Trading Standards department. Over the past few years, Trading Standards departments (which used to be vastly under-funded) have been given extra resources, specifically to deal with online crime. They carry out their own investigations and then involve the police when necessary. (The police can easily refuse to help an individual eBay customer. It's far harder for them to refuse to help Trading Standards, especially when they've got evidence of a major scam).
You can find contact details for your local Trading Standards office by entering your postcode here:
http://www.tradingstandards.gov.uk/
Chris