Gents, officially you CAN'T do it without an agent (up to now, later I don't know).
Someone may be let go by his fortune, as you said. But the chance is very narrow and tricky.
The motorcycle and car price are pretty much different between Vietnam and Cambodia, and the custom doesn't like us to bring our motorcycle, it could be assumed as motorcycle smuggling.
And more, Vietnam and Cambodia yet to sign the transportation agreement (Laos and Vietnam did), so it's really hard to play with.
You may try Laos' borders, they are more friendly, as the agreement is there.