Turn down client or learn?

You could always try the honesty route. Say "I've never done an ecommerce website before. It would be a great learning experience for me and I'd love to build it with you. You'd only be billed for working hours not research." Something along those lines.

I mean, either you're going to be doing the research on how to build an ecommerce site in your spare time and you're not going to get any money anyway. Or you're going to do the research during the project and wont get paid for the hours you put in on research but you'll still be making money throughout learning something new that will eventually make you more money. I'd say those are your 2 options. Anything else seems a bit unethical in my opinion.

I'm big on a quote attributed to Picasso: " I am always doing that which I cannot do, in order that I may learn how to do it"

That said, Just how far out of your wheel house are you being asked to go? Maybe more details would help.

When I was in my early teens, I would take on a lot of jobs from clients, many jobs of which I had no idea how to do. My mentality was that I could do anything with enough resources at my disposal, via the internet.

I learned a ton. I would spend many hours more than I'd admit to the client just trying to figure out a solution. And if I happened to hit a wall, I subsequently met a lot of good people online whom I could ask for help if I got really stuck.

As a personal note, the downsides to this were plentiful. Being a sort of pseudo developer, I never built proper foundations for any sort of language, CMS, or framework. I would use older solutions for newer problems, and it certainly got messy. The moral issue was that I would be lying to my clients every time I told them that I knew how to solve their problem. In return, they would receive a decent to poor value of a solution.

Eventually I became better at solving issues. By looking at more of other people's work, I began to realize that there is a lot of bad solutions out there, and that even my "figuring it out as I go" mentality was sometimes improving upon these other people's work. What I feel, after seeing as much as I have, is that it really boils down to how much the client is willing to paid. I was paid no large sum, $50 was common for 12 hours of work for some time, and they in turn received what their budget could pay for. But I made it work and they were happy.

Apologies if this seems like a random tangent, I'm not sure if this is of any interest to your question.

My biggest advice to someone figuring out a solution "along the way" is to research potential solutions before committing. Really research it. Decide whether the time you have to research is appropriate for the client's project. At worst, if you miss out on the job, you've learned a lot about specific solutions that you'll more than likely use later on. Or you eat nothing for the week, depending on the situation.

I just successfully completed my first ecommerce project. I had no prior experience, but I wasn't going to turn down the job opportunity. I read all about it, and got help from friends along the way.

Are you looking to make an ecommerce website? Why not use something easy like woocommerce?