You already have the hard parts. You know your stock, your prices and your customers. What a marketplace adds is reach, and what it asks for is records.
What does not change
Your buying, your pricing and your judgement about what moves. Nothing here replaces knowing your trade.
What changes
Every sale has a record. Where a counter sale might be a note in a book, a Metzee sale has an order, an invoice line and a deduction you can read. For most shops that is the genuinely useful part, separate from the selling.
The money arrives later and in one place. Not the till at the end of the day, but a balance that becomes withdrawable 24 hours after an order is delivered.
Deadlines are explicit. You have 48 hours to dispatch unless you set your own. A shop that is shut on Sunday should say so in that setting rather than discovering it on a Monday.
The one thing to get right first: stock in two places
This is where shops come unstuck. The same item on your shelf and in a listing can sell twice in an afternoon, and the second buyer gets a refund and a bad impression.
Pick one of these and stick to it. Either keep a shelf that is only for Metzee, or list in small batches and remove a listing the moment the item leaves the counter. The first is less work and far less error.
Running a show from a shop
A shop is a good room for this: you have the stock behind you, the light is already arranged for looking at things, and you are used to answering the same questions out loud. A twenty-minute show at a quiet hour costs you nothing you were otherwise using.
Questions people ask
Do I need a GST number?
That depends on your turnover and what you sell, not on Metzee. It is the same question your accountant already answers.
Can I keep selling at the counter?
Yes. The only real constraint is not promising the same object to two people.
Is it worth it for a shop with regulars?
It reaches people who are not within travelling distance of your counter, which is the whole of the argument. If your stock only makes sense locally, offer collection and keep it local.
More: how to sell on Metzee, selling part-time, keeping stock straight, can I get a tax report.