The "target=*" macro condition only supports (a) unit tokens like "player" or "focus", or (b) names of characters in your current party or raid.
You could try this:
Code:
/cleartarget
/target Auctioneer
/click [exists] MacaroonButton70
However, a better solution would probably be to modify whatever macrotext code is on MacaroonButton70 to only run if the auction house was open. If you need help with that, or aren't sure if it's even possible, you'll have to post the button's macrotext.