Am I Still Allowed to use a used Macromedia/Adobe Flash License?

If the CD was already registered before or the CD was sold or given to the thrift store by a previous owner, then yes: a license transfer is required. Which is impossible to do for the OP, since they do not know who the previous owner is and both parties must complete that license transfer form. 

However, if the OP’s Flash 8 installation CD was never used nor registered, then a license transfer might not apply here, correct? Because this CD might have originated as an unsold copy from a store or warehouse. 

Not very likely, but still possible in this case. If so, the thrift store becomes the point of purchase for the OP, the OP is the first user / owner of this copy/license, and they seem to be in their right to activate it without the need for a license transfer.

https://helpx.adobe.com/download-install/apps/licensing-activation/serial-number-redemption-code/find-serial-number-purchased-store-online-retailer.html

Of course, whether they still can without an existing license server? Probably not, unless they use a serial number that doesn’t require online activation.