I work for a large national department store as a sales associate. My mother wants to open an antique booth at a flea market and/or booth a swap meets. She wants me to help her because I have a degree in merchandising. All goods sold would be used and old. Is this a conflict of interest? I would not be paid for my help or deal with any of the same vendors. I wouldn't be a sales person at the booth either. I am worried to even ask at work because she will be opening it and wanting my help regardless of what they say. Could I get in trouble for this?

Unless you have a contract with the retailer that prevents this, I don't see a problem.

Unless you intend to set up in direct competition to your paid employment, there shouldn't be an issue.
If you have doubts you really need to address them with our own HR people.