Selling a product from modified free plans?

I found some plans on the interwebs that I built and modified a decent amount. Since building it, I have some interest in a few people wanting to buy it.

What are the implications. I sent a message to the person I found the plans from and haven't heard back in a month. There is no copyright or trade mark or disclaimer disallowing commercial use.

I mainly use their movement for swing out drawers, the overall design is different but there are only so many ways you can make a box look different.

I'm assuming since I'm mostly using the joinery design and the mechanism for drawer movement, I'm ok but wasn't sure.