A signature in the 3Dsellers Helpdesk is a block of text, such as your name and shop name, that belongs to one selling account. Create it on Helpdesk > Templates > Signatures tab > Create Signature.
A signature is not added to your replies automatically. It appears only where your template contains the {signature} dynamic field, and it is the signature created for the selling account of the ticket's order.
Full steps, the dynamic fields and the Signatures dialog are in Helpdesk Templates: Saved Replies, Fields, Signatures. See also Helpdesk Replies: Undo, Reply & Resolve, Schedule, Snooze.