Identify the major assumptions when doing sales forecasts.

There are at least two ways to answer this question,
depending on whether you are doing a sales forecast for an existing business or doing
one for a proposed business.

An existing business generally
bases its sales forecasts on its sales figures for a previous period such as the
previous year.  The people making the sales forecast then make assumptions about the
likely changes in sales figures.  In that case, the major assumptions are A) that future
sales can in some way be predicted by past sales and B) that future sales will be
higher, lower, or level (depending on what the people assume) than previous
sales.

A prospective business generally bases its sales
forecasts on the performance of other businesses in the same industry and market.  In
such a case, the major assumption is that the new business's sales will be similar to
(either the same as, or some given percentage of) the sales made by its
competition.