Nope !! Someone selling somethiong I might want, more than one person can show their interest here. It is done every day I see people line up one after another to buy" if soinso doesn't buy I will "
In a for sale ad yes, not in a wanted ad.
So not cool. Write a message, “I’m interested, next in line if he doesn’t want it.”
What your doing is simply sneaking in and robbing this guy of a part he posted a want as for.