Your answer in Telrgram chat was: "Depends on the API, but yes, most times we use the "buy" price, as thats the price users can expect to get when trying to sell their coins".
My opinion: "I think your method is ABSOLUTELY WRONG !
During hard dumps we can't see the actual situation".
Summary : 1) Last Traded Price
2) choose a price-tracking method (Last/Ask/Bid)