It is easy to connect multiple eBay accounts to Magento using Codisto LINQ.

Connecting the first time is a simple as following this article

Once connected to eBay, and on the settings screen, you will see settings for your eBay Account that you have just connected. (See below).

* Note - If you do not connect to eBay initially you will not see the Manage eBay Accounts / Marketplaces option.

To Connect an additional eBay account please click "Manage eBay Accounts / Marketplaces"

You will now be presented with your Store Views and current eBay configuration including options to 

• Change eBay Account for a store view
• Disconnect an eBay Account for a store view
• Change the eBay Marketplace that is currently selected for a particular store view
• Add an eBay Account to a store view
• Add an eBay marketplace to a store view

Select "Add eBay Account"

Proceed to follow the instructions listed next to "When the eBay Sign in form appears, enter your eBay user ID and Password and click Sign In" here

After you have entered the details and been granted application access to Codisto for this additional account you will be presented with a screen such as the following.

The new eBay account will now appear at the bottom of the Store View that you clicked "Add eBay Account" for (In the example above Default Store View).  At this point, you may change the eBay Marketplace that this new account lists on for the selected store view or click Save.

Once this process has completed you will see the additional account that you just connected listed in the eBay settings (as per below).

Now that you have the additional account listed, to specify which eBay account to list on, click the down arrow next to the listed eBay ID at the top of XpressGrid.

Now select the eBay ID you would like to use to list.

After selecting the desired eBay user ID, the XpressGrid will reload and you can proceed to list.