Free shipping where possible. Minimal shipping otherwise. Competitive prices. Fast shipping/order fulfilling.
Plenty of pictures, description of product. Media presence if possible (get on forums like this and be available)
Easy (free) returns when you are at fault. Establish a brand rather than selling generic lights.
It’s just like any other business really. Think about what are the considerations that drive your own purchasing decisions. Think about why you respond to some ads on Ebay and why you pass on others.
Take the uncertainly out of a purchase decision on the part of a potential customer…“Am I getting the best price, will they ship quickly, do the pictures and descriptions answer all of my questions, are there reviews of the product available, does this seller have a good reputation.
Forget “free T-shirts”. Many people don’t want another T-shirt and they know they are actually paying for it one way or another. Many people don’t want to walk around with an ad for your company on their clothes.
Think of the profit margin but think longer term. Be competitive but don’t give away something for no reason and something that might not even be wanted. It’s OK to sell T-shirts. The profit margin is good. If you develop a good reputation people might wear clothes with your name on it.
Figure out what the higher profit margin items are. Be competitive on the lights as people will be checking around for the best prices. Offer batteries with better profit margins. They aren’t going to shop around for batteries if they are buying a light from you (as long as your battery prices aren’t a ripoff).
Put work into your website. Don’t just list items and then say “call us” or don’t have a quirky website. If you have a “contact” page with an email address, respond quickly.
Do research as to what flashlights are popular, have the best interface, only carry those. Don’t carry crap. Don’t try to carry everything. Don’t bombard customer’s email with ads just because they purchased something from you. Be selective in that regard.
Carry a few items that are hard to get. Do something original. Anyone can order anything from China if they want to wait. Make sure they don’t have to wait from your shop. The best marketing is a satisfied customer and the cheapest cost of acquiring a customer is to get a return customer.
The biggest problem when someone has a school project like this is when they want to “fix” a problem with a hobby they don’t understand. I get that you already are interested in lights so this probably doesn’t apply to you but in every hobby that I have some computer programmer always wants to sell a program to “help” that hobby. It’s always fixing a problem that doesn’t exist or that they don’t understand.
If running a business were that easy, businesses wouldn’t go out of business so easily.