I've never been a retailer but even with my small brain there's a few things I'd have to get right first.
1. I'd need to source my product at the cheapest price possible (and protect the details of where I got them).
2. I'd need to factor in the postage costs including taxes/import duty
3. spares availability and warranty
4. lead time and storage facilities
5. Staff? outgoing postage
6. Income tax/vat
7. profit margin
8. competition
To sell it it would need to be different/better than the competition to ask decent prices or cheaper than the equivalent which means selling in bulk.
To be honest, just cleaning windows has a lot of appeal for me