Which free theme should I use?

Try not to overthink this decision. You are looking for a simple, professional looking theme which will act as a great template for you to get your MVS set up quickly.

For inspiration, you could view some of your competitors websites and see if there is a particular style of website that is popular and therefore resonates well with your future customers. Then pick a theme that is most similar and that you are happy with.

The available free themes change regularly but the 'Dawn' theme is currently available and a professional and smart looking theme and will suit most ecommerce stores.

Browse the free themes that are available and check out the demo stores that they provide so you can get a good feel for each theme. Then go ahead and pull the trigger. You can change your theme easily in the future when the time comes for a website upgrade. At this stage you should just focus on creating a functional store that you can use to prove your professionalism to suppliers and make sales with. You can create very profitable stores using a free Shopify theme.

Here is the link to check out their free themes again - https://themes.shopify.com/collections/free-themes