My advice would be to respond with “No” and to then block this buyer.
Firstly, 15 designs for $15 is a dreadful rate. It’s a dollar each. Your potential buyer is looking to abuse you with their price. Remember, in most developed cities a takeout coffee is $4 or $5. Imagine each of your designs being worth a few sips of a cup of coffee. Then imagine someone in another country laughing at how they got you to work so cheap.
Secondly, you appear to be ignoring a massive red flag. How dare they state “if the designs are not up to the desired quality, I will cancel”. Apart form being rude, they are sending out the signal that they will likely cancel whatever the result and still keep your designs.
To answer your question is it a spammer or real, I would say it’s real - but believe me, this is the type of buyer you do not want to work with. It will end in tears.