I used a debit card with a small amount on it for Intelius. I had never used the card for anything else, and to my dismay there were several charges for purchases over the internet that I did not make!!!!
There's a reason why they don't accept PayPal- because they wouldn't be able to steal people's information.
There is no reason for such a site not to use PayPal if they are honest. I would not recommend using their site unless until they do or any other people search site that does not, as many of them seem to be run by the same company.