In the united-states, the parties are generally free to agree to a different rule, but by default, the rule under UCC Article 2-401 is that:

title passes to the buyer at the time and place at which the seller completes his performance with reference to the physical delivery of the goods.

It is therefore not clear whether Bob is the legal owner of the thing yet. If this transaction unfolds at Rob's garage sale, and Bob is already holding the goods, it seems unlikely Rob as any futher delivery obligations, so the title has likely transferred.

But if Bob is reaching out Rob listed This Thing on Facebook Marketplace for for $1,000 with an offer to deliver it to the buyer, then title would not transfer until Rob makes delivery.

In the united-states, the parties are generally free to agree to a different rule, but by default, the rule under UCC Article 2-401 is that:

title passes to the buyer at the time and place at which the seller completes his performance with reference to the physical delivery of the goods.

It is therefore not clear whether Bob is the legal owner of the thing yet. If this transaction unfolds at Rob's garage sale, and Bob is already holding the goods, it seems unlikely Rob as any futher delivery obligations, so the title has likely transferred.

But if Bob is replying to Rob's listing of This Thing on Facebook Marketplace for $1,000 with an offer to deliver it to the buyer, then title would not transfer until Rob makes delivery.