It's hard to say when the Barnes and Noble manga sale occurs. Sometimes it's announced on their social media or in-store flyers. Keep an eye out for those!
I'm not entirely sure as it could be various things. It might be a side story related to a story about love that is somehow for sale, perhaps a relationship that has a transactional aspect to it in the context of the story.
Specialized manga websites or forums dedicated to manga enthusiasts might have listings or recommendations for where to purchase 'A Lull in the Sea' manga. You could also ask for suggestions from fellow manga lovers on such platforms.
There's no way to tell without more information about this 'love for sale side story'. It could have a whole range of characters depending on what the overall theme and plot of the story are.
The novel " My Youthful Love Story is indeed problematic " had been completed and was already on sale. The novel was published in Japan in April 2013 and published in July 2017. The novel tells the story of the protagonist Haruki Ohara's emotional entanglements with various girls in high school life, as well as the love story between him and the heroine Yukino.