Is there a way to scrape the Seller URL (or ID) from an ASIN ?
Example
Asin:
https://www.amazon.es/dp/B07DNH3993/
Seller:
https://www.amazon.es/gp/help/seller/at-a-glance.html/ref=dp_merchant_link?ie=UTF8&seller=A9PG8O0EXLSVE&asin=B07DNH3993&ref_=dp_merchant_link&isAmazonFulfilled=1
You can try this XPathOnUrl request (not sure if it works for other amazon sites):
=XPathOnUrl(A1;"//a[@id='sellerProfileTriggerId']";"href")
It's an internal URL so you need to concatenate it with the domain. See the full link on the website and try to build the same.
Is this the reason for the spill error ? Why do I get the same internal URL twice ? Thanks !
No idea what you mean with "same internal URL" but now you have everything to figure this out. Check what you get from original HTML and build good XPaths 
This is what I'm trying to do 
I was referring to this:
And with more URLs I get the Spill error, so I guess it's related...
Use "@" in the beginning of the formula to force Excel to only return the first match (to avoid #SPILL!).
=@XPathOnUrl()