Ok, back to real life. It is great to recycle old seed starting trays, but they need to be sterile. Old trays can harbor pathogens that can dampen off young seedlings.
Here is what you "should" do.
After you use the trays initially and transplant seedlings into your garden, clean your trays using the following steps:
1. Shake out loose dirt.
2. Rinse off with garden hose.
3. Wash with 10% bleach solution or soapy water.
(some gardeners soak trays in a tote or kiddie pool of bleach water)
5. Let trays completely dry in the sun.
6. Store in plastic garbage bags until ready to use.
Whenever you choose to clean your trays, your seedlings will be glad you did.