Signing a message means that both the sender and the receiver will sign a message respectively using their ECDSA private key. For example, if they sign a message stating:
"XX is in control of ADDRESS", you will know that XX has the private key since it won't validate unless it has been signed with the private key that corresponds to the address. Thus, you will be able to tell if the transaction has really occurred if there is a transaction between the two addresses.

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Address reuse is not bad, per se but it would decrease on their privacy. It is advisable for the seller to be able to generate a separate address for each transaction anyways. If all else fails, you can ask the seller to get the buyer to send Bitcoins at specific amounts, (0.00201928 vs 0.00201900) for example. The downside is that the seller has to update you about the amount of Bitcoins that will be sent by each buyer.