How to forecast for a new business

I am putting together a business plan for a start up small business. will be a consignment and gift shop offering gently used apparel, accesories and home goods (including furniture) as well as items created by local artisians. My question pertains to forecasting for the business, since I have no idea what people will want to consign, how many customers or average purchase will be I have no idea where to start. Can anyone offer any advice?

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