Under each marketplace you will see a check box entitled 'download product catalogue' or 'download products'
When you first connect your account please enable this box and select the button on the top right hand side that says 'download product catalogue'. This will begin to import your entire catalogue from the channel. It will generally take around an hour to import your full product catalogue.
If you leave the box 'download products' enabled then this will run a feed every 24-48 hours which will import any new products into Selro.
A new product is classed as any SKU on your channel that isn't already in Selro.
When these new products import if the same SKU number is already in Selro then the channel product will sync with the existing Selro product. (If you have stock synchronisation enabled then your channel stock levels will be updated with your Selro stock levels. Inventory-> inventory synchronise)
Please note if you have merged products together leaving this box checked (pictured below) can cause your products to unmerge as they will be reimported as new products.