https://forums.backpack.tf/topic/6033-rules-for-unusual-price-suggestions/?do=findComment&comment=674608

Our price moderators have no access to Mannco anymore, and along with the recent changes that do not allow the viewing of their sales graphs without logging in, we are no longer able to verify Mannco sales. Unfortunately, that means we can no longer allow trades that occur on Mannco for price suggestions.

find the price of keys at the time of your purchase and divide 750 by that amount - it should be one number, you can’t source a range from one sale