Consumer demand: a function of present endowment?
Although it's often left unstated, demand for a good is measured as quantity per unit of time.  Broadly, the longer the time period over which demand is measured, the smaller the (proportional) effect on demand of the initial endowment.  Whether you have a soda in your fridge one morning may make the difference between your demand for soda that day being $0$ or $1$. But over the next year, if you often consume sodas, it may make the difference between your demand being $n-1$ or $n$, only a very small proportional difference if $n$ is large.  For this reason, even at the individual level it can often be appropriate to ignore endowments as a determinant of demand for frequently consumed goods.

At market level, a further consideration is that the effect of endowments may average out over many individuals. At a certain time, for example, A has a recently purchased washing machine, B's washing machine is in mid-life, and C's is old and needs replacing soon.  In the absence of changes in income or major improvements in washing machines, it may be that in any year approximately the same proportion of people have a demand for a new washing machine, so that both the aggregate endowment of washing machines and the market demand for new washing machines are approximately stable.  Again, therefore, it could be appropriate to ignore endowments.

But there are also many situations where neither of the above apply and it is important to consider endowments as a factor affecting demand.  These include: in consumer markets, take-up of goods embodying new technologies; in housing markets, the size and condition of the existing housing stock; and in commodity markets, stocks of for example wheat or copper held by commodity traders.