The vendors sold baked sweet potatoes by shouting 'Sweet Potatoes! Sweet Potatoes! It's 13 ri (old Japanese unit of distance), 4 ri more delicious than 9 ri!,' which is a wordplay meaning that sweet potatoes are more delicious ('more' is synonym of '4 ri') than chestnuts (synonym of '9 ri'.)
---「やきいもーやきいもー栗よりうまい十三里ー」栗より美味い十三里と言う文言は「栗」を九里と「より」を四里とを足して焼き芋のことを十三里というなぞかけ言葉である。