Mistakes and Misrepresentations

In a previous segment, we discussed a case where a shliach did not carry out his instructions as intended, and we said that the transaction is not valid. In the case discussed, where the shliach rented an apartment on behalf of the meshalayach that he did not want, does the shliach have any liability to the owner of the apartment who is going to lose his renter because of his actions?

Reuven bought an item from a store, and when he arrived home, he noticed it was defective. He returned to the store with the item and receipt and told the cashier he wanted a refund. The cashier replied that he is just the middleman and Reuven needs to call the original seller and discuss it with them. Can the buyer insist that the seller he dealt with take personal responsibility?

A fellow sends his neighbor to buy something for him at an auction; however, when the neighbor arrived at the auction, he decided that he wants it, so he bought it for himself. When he came home with the item, the one who sent him insisted that the item belongs to him because he sent him as a shaliach to obtain it for him. Is he correct?

In a previous segment, it was discussed that if a shaliach changed his mind before the purchase and instead of purchasing it for the one who sent him, he purchased it for himself, then the item would belong to the shaliach. How do we know if he changed his mind before or after he purchased it?