Anyone?
You can use a block explorer such as
https://blockchair.com/ to see if those transactions actually happend.
But even then, you should be cautious.
He could've simply taken/snapped those transactions from a node (depending on his business model). ( A lot of HYIP sites do this. (I do hope that's
not what you're trying to buy here!))
If you really want to make sure that the "company" made those transactions, ask them to sign a message from the adress that the output was made from.
https://bitcointalk.org/index.php?topic=990345.0(And then ofcourse verify it.) Although, if you gotta ask this, i'm pretty sure something is fishy.
What exactly are you trying to buy? An exchange? A gambling site? If so, you could check their traffic sources using
https://www.alexa.com/siteinfo or SimilarWeb as well.