Are they buying the shop too?
If they're buying the web site and that generates 80% of business, does that mean they're also buying the shop or are the original owners keeping 20% turnover for themselves?
I know setting up an ecommerce web site would cost a bit, but are they absolutely sure it's worth spending £70m on an existing site? Don't see Tesco's doing that - when they want to do something they just seem to set it up themselves, like Tesco Direct. Noteably, they seem to make a lot more money than Morrisons.