Although it's difficult to fully quantify eBay as a direct competitor to your UK High Street store, there's more than meets the eye in what eBay offers. Established retailers have their own eBay pages where they sell off stock that is discontinued or slow-selling elsewhere. As long as you have a fix on the costs then there is nothing stopping you using the eBay marketplace for any if your products.
I have spoken many times on this blog about the advantages of having an online offering, especially when you offer Click & Collect as the main way of delivery. This gives your customers access to a range that you may not be able to carry in your shop every day of the week. eBay could be an extension to this - carry your full range, offer larger items as collection only and smaller items for postage. Amazon marketplace is another option for sellers of some products.
As well as individual retailers making use of the technological marketplaces offered by these American retailing giants, there is the question brought up about how this competition affects the future if the UK High Street. My blog is based around how I see the future of the High Street. I can see a massive challenge from online, I can see a massive challenge from overseas companies, such as eBay and Amazon, but I can see a bright future if retailers differentiate and if they can make the most out of online.
It's not a direct choice between online or offline, but the two markets will change. The main man at John Lewis has recently said that customers can't go on expecting to have items delivered and pay the same price that they pay in physical shops. This to me seems contradictory, even taking into account delivery costs online must have much less in the way of overheads.
Overall, I think that if you are an individual High Street retailer you can still stand out and be successful. If you differentiate on quality and service and use online to supplement your offering then this will keep your business vibrant and Click & Collect will help keep people coming to the High Street and keep that vibrant as well.