Give the meaning of "but one's imagination could not have pictured the wearer embarking on---------- ninepence on a carnation buttonhole"DUSK by...

These lines are descriptive of Gortsby's perception of the
elderly gentleman who sits alongside him on the park bench in the "gloaming hour." 
Taking a rather cynical point of view regarding the citizenry of the park at dusk,
Gortsby perceives the elderly man as having only the last vestiges of self-respect as he
no longer can command the esteem of others. While Gortsby sees the old gentleman's
clothers are not really shabby, he does not feel that they are at all
new:

...but
one's imagination could not have pictured the wearer embarking on the purchase of a
half-crown box [two shillings, sixpence/35-40 cents] of chocolates or laying out
ninepence [20 cents] on a carnation
buttonhole

Gortsby does not
imagine that the elderly gentleman is affluent enough (20 cents bought quite a bit in
the early 1900s) that he can waste money on chocolates or a flower for his coat's button
hole.  For, if he were, the gentleman should not be sitting in this
park. 

The reader, however, soon learns that Gortsby is not
quite the reader of people that he imagines himself.  In fact, the elderly gentleman
does return in search of the soap which he has purchased, placing Gortsby in the
position of having been taken advantage of by the young man who has come along and taken
the elderly gentleman's place and given Gortsby a very tall
tale.