Requesting refunds can be the reason for some sellers to blacklist you, as refunds negatively impact statistics and some buyers will unscrupulously request refunds for many purchases.
To add a layer of protection for sellers, each seller can set a Peer Blacklist Network threshold to block customers before they buy a product. This threshold value is used to determine how many other vendors must ban a customer before that customer is also banned from the seller’s products.
This would explain why you aren't able to purchase from a specific seller that has not blocked you directly.
The same concept applies to a retail store when you see a sign on their door that says, "We Reserve The Right to Refuse Service, at any time, for any reason". Therefore, there is nothing JVZoo can do to prevent you from being added to a blacklist. This is something that each seller determines on their own, for their protection.