"The user will send the shipping costs, but how will he ensure that you will not steal that money?"
- The user sends me [cost of item + shipping + small fee]. The guarantee that I won't steal that money is the same as purchasing anything online. That said, investigation into using escrow services and third party review sites is coming quickly. Stay tuned!
"How will you prove the payment address, since all correspondence will be by e-mail?"
- I'm sorry I don't understand the question.
"Can money be deposited in multi-signature wallets?"
- I will provide a unique wallet address used for a single transaction, and only that transaction. I don't see where multisig wallets are applicable but let me know if I'm missing an opportunity.
"The second problem is privacy, how does the buyer ensure that the money that was used for the purchase is legal and taxable and not from unknown sources?"
- Instead of using my service a criminal can take stolen precious metals or crypto, exchange it to fiat at a pawnshop or localbitcoin or something, then use that fiat to buy something online. So basically my service is no different from the status quo, just one less step.
Thanks for your questions.