I work for a company that list small local business from a niche market in a website. Most of these companies have little infrastructure on the internet. So to improve visibility we are adding the Schema.org Local Business in their profiles.
We looked in the schema specification and the google documentation, but it isn't explicit anything about local business ownership. So it is not clear if this structured data allows listing other local businesses instead of your own.
Is it okay to use this schema to create rich snippet cards if you don't own the company? Can this have negative outcome as a Search Engine Optimization?
Providing structured data about other businesses (or any other domain) is perfectly fine. Structured data is useful for the content on your pages, not just for the entities that publish this content.
A consumer (like Google) that offers features making use of this structured data (like rich results) has its own rules for this feature, of course. But even if the consumer would support this feature only for structured data about the author’s own business, this should not stop you from providing the structured data about other businesses.
As a general rule: You provide as much structured data as you can/want, and consumers pick out what they want to use.
To convey that it’s not your own business, you can provide your own LocalBusiness as publisher/author of the WebPage, which is about the other LocalBusiness.
<body typeof="schema:WebPage">
<header property="schema:author schema:publisher" typeof="schema:LocalBusiness">
<h1 property="schema:name">Your own business</h1>
</header>
<article property="schema:about" typeof="schema:LocalBusiness">
<h2 property="schema:name">The other business</h2>
</article>
</body>