Is there any prohibition against county employee's purchasing forfeited property at a public auction? I can't find anything.

What're you selling?

If it's 8-liner stuff, have a look at Attorney General Opinion No. GA-0533. It concludes that 8-liner machines can be sold by competitive bid as salvage with conditions to prevent recurrence of illegal use within the county. Seems to me that could infer authority to preclude county employees from eligibility to purchase, or at the very least require that they expressly agree to remove the devices from the county. Concurrently, it's a general employment rule in Texas that an employer has a right to discipline an employee for off-duty conduct that relates to the employer's business and that reflects poorly on the employer. Possession of contraband or unlawful use of property would seem to fall within that rubric. There is no constitutional right to violate a facially-valid penal law.

These are vehicles and other items; no eight-liners or things like that (that I know of).