There are 3 benefits of using the multiple variation listing -
1. You save time since you list only once instead of multiple times
2. You get to save money. When you listing your items this way you'll pay fewer insertion fees, and get special pricing for pictures with variations
3. It's an easy way of gaining more visibility: Once buyers start purchasing from your multi-quantity listings, your listings can appear higher in Best Match search results.
To learn more about multiple variation listing, please visit this link -
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A completed listings search would be a good idea! It's pretty hard and risky to assume the sizes.
By the way, welcome to SaleHoo!