Join Date: Jul 2015
Location: Carson City NV USA
I did not open the link to a photo you provided - that's a very quick way for a computer to catch a virus. Maybe you can upload the photo directly to the forum page
To answer your question: Yes, this is bad. Rust is like cancer - once it starts, it keeps growing until it destroys the part it's on. So my recommendation is to delay the purchase and wait for the tank to be replaced or take another motorcycle from the dealer's inventory.
Generally, a dealer will not sell (deliver) a brand new damaged motorcycle to a customer since it will have to be repaired under warranty anyway. It might be understandable for a dealer to do so with a new model for which spares might not be available yet, but he should order a new tank from Honda and state (in writing) on your sales documents that it will be replaced at no charge to you.
Before taking the bike with a damaged tank, follow Marc's advice and check for rust inside. If there is any sign of rust - walk away. Rust particles in the gasoline can clog up the fuel system and leave you stranded.