Your eBay store needs to subscribed to eBay's Starter plan at least in order for you to use any integrations, including Trunk, with your eBay store.
If Trunk is complaining that you aren't subscribed to an eBay store plan while connecting an eBay sales channel, it's because you aren't subscribed to an eBay store plan. You can see all the available eBay store plans here.
If you're not sure whether you're subscribed to an eBay store plan, you can view your current active subscriptions on eBay. If you're successfully subscribed to an store plan, you should see an active store subscription similar to this (in this example, this store is subscribed to the Starter eBay store plan):