You can start documenting your project on your blog or Hackaday.io in advance. That will help build interest and a following before you launch your product.
As previously mentioned, you do need to have your product ready to ship before you list on Tindie and take orders as people don't like delays. We require sellers to ship within 14 days or the order is cancelled and refunded.
If you are going to do a small hand-built batch of 10-20, I would still work out the pricing for a larger production run so you can work out your pricing for both batches. You should price anywhere from x2 to x5 your BOM and manufacturing costs.
Also, factor in shipping loss or damages into pricing. It's the seller's responsibility to make sure the item reaches the buyer in good condition. If the item is lower priced, you may just add 10-20% to the batch, offer cheap shipping and resend if any packages go missing during shipping. If it's higher price or a large order, you will want to send tracked and insured even if that's not what the buyer selected.
Let us know if you have any more questions.