If you know who made the pieces, you can always ask them if the new character owner could display the art. Usually (but not always), they're okay with it if properly credited. They also appreciate knowing who has the characters, to keep their description up to date, as they made it.

Other people in art with you...many don't want to be in a picture with random people they don't know, so ask, and if they're not, that piece should remain out of the deal. They likely want it to be with "you."

Technically, all you can sell is the character and art you've produced, so keep that in mind for pricing. Anything you've gotten from others was mainly for your enjoyment of displaying. It's a factor only if the artist lets it be displayed by the new character owner, not just the person they made it for.

If you think you might revisit the character, consider keeping it. But if you're really in a bind, but don't want them going somewhere you don't know how they'd be used...consider private offers, people you know first, then expanding. If you think you'd want them back if they didn't want them in the future, consider keeping right of repurchase in the deal, so you get first opportunity to buy them back.

If you don't want to sell them, you could offer a special price on commissions using those characters.