I've always wondered about this ;)

If I'm to act under the basic premise that psychics are real (extremely unlikely...)...

It would depend on the type of psychic. If we're talking someone with precognition that is picture-perfect, eg. they can SEE the lottery numbers ahead of time, then sure- by all means. But precognition is rarely described as such, it is usually described as being very symbolic and requiring significant interpretation; something like seeing what balls pop up in a lotto draw could well be beyond them.

As long as we're assuming precognition is real, then why not an afterlife or karma (buddhist or western)? Doing this could be seen as 'stealing' the lottery from whomever might have won it otherwise; A precognitive may well see that directly abusing his or her power in this manner could result in punishment in the afterlife or a downgrade in the buddhist karmic cycle, or serious in-life repercussions resulting from western karma. Charging a reasonable fee for their services may carry no such repercussions. Compare hackers: A black-hat hacker might steal information and sell it to the highest bidder for personal gain; a white-hat hacker might instead assist people, companies, and governments with hardening their own networks vs. intrusion. Same skillset, different moral outlook.

Psychics of other types (or multiple types) that lack precognition, eg telekinetics, telepaths, mediums and the like, simply wouldn't have the basic ability to do this.

Whatever they are offering, real or fake, people are willing to buy it. The capitalist way dictates, if people will buy it, sell it.