The GetProductInfo method comes with two parameters: assetId and infoType.
In your script, I recommend always specifying the second parameter. You can easily do this using Enum.InfoType[infoTypeName]. In your case, a shirt would be an Asset. Therefore, you use Enum.InfoType.Asset.
Meaning your code would look like this:
local Asset = game:GetService("MarketplaceService"):GetProductInfo(00000000, Enum.InfoType.Asset)
print("This Shirt has " .. tostring(Asset.Sales) .. " Sales")
The above script will tell you the sales of an asset. The sales a player has aren't really accessible through a script. Not that I know of anyway.
Kind regards,
Rodrigo455