I can recall a popular set of books. The first of which, IIRC, the main character (a boy) swaps things. The title may have even had “swap shop” in it? I half-want to say it was in the “Danny Dunn” series, but then my other-half doesn’t think so…
Does anyone else recall such a book? It was from the 1970’s, or earlier?
Alvin Fernald, I think. Alvin appears in several books, one is Alvin’s Swap Shop, and I think the best known is probably The Marvelous Inventions of Alvin Fernard. The series has a bit of a Great Brain feel to it. Clifford Hicks is the author.