Are the markets being manipulated?

I'm partway through a 1990s American TV programme (htp: Jesse) about the lead-up to, and aftermath of the Great Crash of 1929. At that time, share price manipulation was legal, everyone knew it went on and even the losers came back for more, hoping they would get out in time the next time round. And in the 1920s, buying on margin became possible, so that provided a fatal extra impetus.

You know all this, of course.

My question (and pardon my ignorance) is about the interaction of derivatives and stock trading today. A takes a huge bet with B that the share price of Widgetco will go down - what stops B from borrowing more cash, purchasing Widgetco in time to boost the price before the date of the bet, collects the cash from A and then sells his firm's holding in Widgetco? Even if now illegal (and I'm not sure of that), are there not ways and means?

And are there other tricks to catch the operator who goes long on a share, instead?