It becomes a regression problem. You use ML to estimate the conversion rate for each item. You still have to do feature engineering, but fast regressors like XG

It becomes a regression problem. You use ML to estimate the conversion rate for each item. You still have to do feature engineering, but fast regressors like XGBoost make this much easier. https://improve.ai https://improve.ai is one such scorer/ranker built on XGBoost.