My dad and I had this dispute on what to say when I want to tell someone to use an item well. My dad thinks that "Have a good one" is a way of saying it (but I disagree). I suggested that "Use it well" is a way of saying it, but we both agree that that phrase is both awkward and we have never heard anyone say it like that. Is there another way of telling someone to use an item well after you sold/gave it to them, that is not saying in the context of "take good care of it," but more of "I hope this(these) item(s) serve(s) you well," etc. etc.?
How about "enjoy your object-name" where the expected response would be "I will" or "I hope to" ?
"Have a good one" does work in this context, but you're right that it has nothing to do with whatever you sold them; it's just a generic farewell.
If you sell someone a particular object used to get something done, you might say "I hope that works well for you!" If, instead, it's a set of related objects — a can of paint, a paint roller, and a tray, for example — you might modify that to refer to the activity: "I hope your paint job turns out good!" ("Job" here doesn't necessarily refer to work that's paid for; it can easily apply to someone's amateur home improvement project.) If it's a set of unrelated items, or if they're not really for getting specific things done (for example, a poster of a favorite music band), it's best to avoid anything like those phrases.
In any case, though, if you're not confident in using these specific phrases, it's perfectly safe to use a more general goodbye instead.