Syncing my Shopify & eBay Inventory + use of Duplicate SKU numbers

[Key-Library]

Hello,

Just a quick note before I explain things - I do around 2000 transactions on eBay a month alone. Small profits sold in large quantities. I assume any paid for services are going to cost based on quantities sold, which is annoying and a disadvantage for me! 

I currently run an eBay store which is going great, but after much demand from customers for different types of products outside of the platform, faster delivery etc, I have decided that a Shopify store is going to be the way to go.

The two import things for me is;

• Use of multiple SKU numbers on Shopify
• Inventory sync between eBay and Shopify as I don't fancy doing this manually (disaster waiting to happen, lol).

SKU Numbers;

So, this is where it gets a bit messy for the stuff that I sell. I sell Auto Locksmith supplies: Car key cases, remotes, blades, and more. So, one product and it's SKU will be suitable for maybe 6 different vehicles and brands. For example, one key case will be suitable for a KIA car, but also suitable for a Hyundai car, and a Toyota car, as well as different models. 

As an example; I use a SKU number on eBay 6 times, but on Shopify, I will want to use it only twice more. That's 8 times in total between both platforms. Is there a solution for all of these SKU numbers to sync inventory levels without any issues between the two platforms?

On Shopify;

eBay allows duplicate SKU's right from the get-go. If I have 500 of one case, I will split it between the 6 of them and play around with numbers as I go... Shopify does not allow duplicate SKU's, so I will need a solution for this too.

I am trying to do this as cheap as possible whilst I try this expansion to Shopify as I don't know how well it will go.

Thanks in advance!

[Key-Library]

Forgot to mention, I also use different titles on each platform for the same products... as titles on eBay are not suitable for Shopify.