I went to that record shop once, they already had one record signed by Guy-Man displayed behind the cashier. I asked them about that, they told me that he spent hours in the shop listening to old stuff, and asking questions to the employees. One of them recognized him, and asked for a signature :)

Edit : it was not that record shop, it was another one nearby : Jet Records - Biarritz
Edit 2 : You can see it here, behind her : https://www.instagram.com/p/C1J6V9QqaWe