Can ownership be transferred?
Yes. You do not have to be in possession of property to transfer ownership of it.
Can C pursue A in order to get those items back for B and/or
themselves?
Either B or C can sue A in the place where A resides or the in place that B's belongings are located, which may be the same place. Citizenship is (almost entirely) irrelevant in this situation.
If the amount in controversy is less than $75,000 the issue probably has to be tried in a state rather than a federal court, as there is no obvious issues of federal law presented by the case.
There is probably more than one state court with jurisdiction over the case if the value of the property is modest. Usually, either the general jurisdiction state trial court of the appropriate county, or the limited jurisdiction state trial court which is not a small claims court, or both, would have jurisdiction over a case like this one.
But many small claims courts don't have jurisdiction over claims to regain physical possession of property, and instead only have authority to issue money judgments.
who's to say that it isn't just a case of "he said/she said?". C could
lie and say that the items are theirs just to have leverage to make A
give them up
So what?
Statements of witnesses subject to cross-examination in open court that are made under oath are evidence. Cases are routinely resolved based upon witness testimony with no other evidence.
Courts handle cases where there is an irreconcilable dispute of facts because someone lies all of the time. The parties try to present evidence to show that they are the more credible person and that the other person is less credible, and the judge or jury does his or her best to decide who is more credible.
Footnotes on terminology
A lawsuit to regain physical possession of tangible personal property was historically called a "replevin" action and is still often called that in U.S. practice. The more modern "plain English" term for this kind of lawsuit is a "claim and delivery" lawsuit.
