Should I use We/Us instead of I/Me even though I'm the sole owner/manager of a very small online business, to sound a bit more professional?

Sorry if this is a stupid question

It's not stupid at all. It really depends who you are working with and your niche.

Most clients will not have any issues with you being a one man team if you are professional and you deliver.

If you have corporate clients, presenting you as a team may sound more professional but you need to take into consideration that you're not telling the truth and they might found out.

Yes, I forgot where I read, the book says, always use "we" instead of I or me.
It sounds more professional, it sounds like every decision is based on the organization.

Even though in reality you're just a single guy working from home with pajama.

Short answer is yes. It sounds professional. Also overcome the imposter syndrome if you feel like it.

No. I always ask if they have MPD if they’re using ‘we’ being an individual.