The space within the outer sarsen circle could easily have been used for a livestock market. A possibility was to use the sarsen horseshoe to auction livestock.
Also the distance between the bluestone circle and the sarsen circle is sufficient for cattle. This is very important. Thanks to the stones, the space between these circles could be divided into (temporary) small pens or stalls (maybe 30 minus at least 1 in order to provide an entrance to the interior of the bluestone circle). Even the preceding Q-and R-holes could be used to give pieces of cattle a separate space but then in another way. Even without using additional wooden structures (easy to make) they could use the bluestones simply to fix cattle. I have read that many bluestones didn't stay entirely upright not so long after they got their definitive place. Did this have to do with the force of cattle?
I recall that the outer sarsen circle provided a solid defense against wild animals, especially aurochs. If the people of Stonehenge had a signalling system between high points of the area maybe even as far as Beacon Hill, the danger of passing aurochs could be anticipated on time. Such a network to signal danger is perfectly normal to defend a society and probably is about as old as war itself, certainly war between (semi-)settled societies.
The danger of aurochs (which not only came into the neighborhood to graze but in part probably also to mate) diminished globally over time. Maybe the danger was already seriously less at the time the Y- and Z-holes were made. These later holes could have been added to provide more spaces for livestock during markets.
Of course, buyers have to see the livestock very well before they buy. Several cattle markets didn't have a roof in the past. You can find paintings, photographs which prove this on the internet. I know that some people think Stonehenge had a roof and some proposals of how to achieve this, maybe aren't ridiculous at all. Providing sufficient light inside may, however, been a serious problem ...
Maybe there were other ways to partition and use the space within the sarsen circle for a livestock market.
The space within the sarsen horseshoe (containing the bluestone oval, later bluestone horseshoe) is, however, largely sufficient to auction livestock. Auctioning livestock normally saves time and increases the price. All this may help to explain why Stonehenge was build the way it was and why it was worth the effort.
Docteur en Arts et Sciences de l'Art
P.D. I repeat that I do not exclude other functions, even ritual and ceremonial ones. Do not forget also that the great Trilithon provided orientation and an approximate watch from wherever it could be seen enough clearly.