the biggest risk is if the dev subsequently removes your udid as you won't be able to install any more betas. Also, as the dev is breaking their agreement with Apple they could lose their dev account which again would leave you out in the cold with newer betas.
Apple got a nice boost in their so-called "developer count" last year when people discovered that anyone could pay $99 to be a "developer" and then sell 100 spots for a 3.0 beta download for a huge profit.
There's a market building up on eBay for them since 4.0 was announced. I've considered buying one each for my iPhone and iPad but my ISP has dropped our speeds significantly for a few days so there's no point right now as it'd take damn near a day to download the firmware. Gives me some time to consider it I suppose.