Requiring a reasonable fee from your applicant to conduct a credit check is a common and legal practice. It is commonly accepted that $50 is a reasonable deposit.
However, if you accept the applicant after the credit check, you are obligated to pay back the full amount to the applicant, either directly or by deducting it from the first month’s rent.
In addition, if you do not accept the applicant, you are also required to return the deposit. There is, however, a situation in which you can keep the deposit: if you contact the applicant to accept him/her, but he/she declares that he/she is no longer interested. The deposit can’t be claimed by the applicant.