The sale period of item A and the sale period of item B overlap for a short time.
There's a slight overlap in the end of the sale period of item A and the beginning of the sale period of item B.
ーThe sale period of item A and the sale period of item B overlap for a short time.
「商品Aの販売期間と商品Bの販売期間は短期間重複している」
sale period で「販売期間」
to overlap で「重複する・かち合う」
ーThere's a slight overlap in the end of the sale period of item A and the beginning of the sale period of item B.
「商品Aの販売期間の最後と商品Bの販売期間の頭には少し重複がある」
There's a slight overlap ... で「少しの部分的重複がある」と言えます。
ご参考まで！
カナダ 
