Hello Conor, thanks for your interest and for the compliment.

Purchased spots will remain at the top with the available spots listed below.

So, for example, if there were 3 advertisers then those spots would display and rotate randomly within the top 3 ad positions while the "advertise here" default ads would fill in the lower 7 ad positions.

Each ad is static in that it is displayed on every page view where the 125x125 ad column on the right is shown, but the ad spot positioning within that column is random, other than any unsold spots which would fill in the lower positions.