Whether you actually sell them or not still requires that you trade on a business account. You can have more than one eBay account for selling, you can open another and sell your own personal items on that one.
Thanks for the listing details.
The first listing, the one with the multi buy, hasn't been set up correctly - eBay don't allow you to offer a 'choice' in the description and have the buyer message you with what size they want - you need to set up a variation listing and have a dropdown menu for the sizes and colours. Then buyers can select the exact item they wish to buy and there's no chance of the same item being bought by different sellers.
You also need a variataion listing for the grey pyjamas.
You need to use eBay's new listing tool to set this up. If you're already using eBay's new unified listing tool, at the very top of the page you’ll see the Photo & Video fields when you're setting up your listing.
Can you see that at the top of the listing form you're using?
If not, go to Seller Hub and then click on the blue 'Create listing' button that's located top right:
https://www.ebay.co.uk/sh/lst/active
Put in your title, then when you start to list your item you'll see the Photos & Video field at the very top.
The option to set variations is located between the Item Specifics and Condition sections, it'll appear once you've selected to list in Buy It Now format in a category that supports a multi variation listing.
If you still do not see the variation option between the Item Specifics and Condition sections, click on the 3 dots on the upper right hand side of the listing form and then select the settings gear icon and click on the variations toggle so it turns blue.
Multi-buy can be set on any fixed price multi-quantity listing, but not a variation listing.
