When saving a schema or previewing a form you get an error such as this one:
Generally the problem is that your XPath is wrong. In this particular case, the XPath is referring to the extension schema namespace rather than to the correct namespace, i.e. nms:recipient.
Correct your XPath, the syntax in a transaction schema for a buyer link would be:
<element label="buyer" name="buyer" target="nms:recipient" type="link"/>
even though the Recipients schema might have been extended.
Note that the link to the product in this transaction schema does use the custom namespace, because neo:product is a new datatype, not an extension:
<element label="product" name="product" target="neo:product" type="link"/>
Review the XPath rules, and remember that an extension schema is not a new datatype. Rather it is a way to keep schema modifications isolated so they are not lost during upgrades.