I don't think that anybody will complain if
you refuse to end the auction early. Just tell
the person that you prefer to take your chance
and let the auction run to its end.

Now, if the person offer a high buy-out price,
it's up to you : maybe you can get a higher
price by leting the auction run or maybe you
will get a lower result...

Cheers,
Yann Stettler