I'm looking at a company who owes me some money and I'm trying to ascertain
it's liquidity.

When a company purchases shares from one of it's shareholders, would the
valuation of the company's assets increase in line with the purchase? Or
would I expect them to stay the same?

In this case the company assets have stayed nominally the same, but the
shareholder's funds have decreased by a similar amount, which of course may
reflect the business itself.