Although it might not look all that much, there's quite an impressive amount of database supporting the system, which as it grows will need to cope with many products and "listings", or items for sale by different sellers. Changes to listings; price updates, new items, items being sold etc. have to be reflected on the website in a timely fashion while at the same time not having everything fall over under the load of a very busy website.
The past couple of months in particular have been a bit of a crunch-time, an awful lot of different things needed to happen all at once to get the project ready on time, which was governed by having to have it ready and tested both functionally and for performance before the run-up to Christmas (which like most other retail businesses, is by far Play.com's busiest time).