The most common and most likely answer to this crossword clue is the 7 plus 5 letter word SELLING PLATE
I've seen this crossword clue in The Times
S
E
L
L
I
N
G
P
L
A
T
E
Buddy explains!
The straight part
We know that "a pre-purchase event" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a pre-purchase event = selling plate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.