No, basically there are software add-ons to Shopify that let you import an AliExpress seller's product into your store.

Your customer never sees the original seller's listing, and therefore you can charge any price you want.

So, as long as you charge a price that is higher than the raw cost + S&H, you can then collect money from your sale, and use that to purchase the item from the AliExpress vendor (this is automated). Part of the automation process fills in your customer's mailing address, and the vendor fulfills their order directly. (So you don't have to hold any inventory, you're just the go-between.)

Your AliExpress vendors will know what's going on, but they expect and anticipate that people do this. It's how they make money.

Essentially, the service you provide is marketing, design, and salesmanship. They provide the product.

Thanks for the info Question:

how do you find the vendor in AliExpress ? do you need to talk with him before ?

You can--but AliExpress has a good reputation system. I mostly use that.

And if a vendor burns me, I remove their products and look elsewhere.

Thanks for answering , is it your full time ? or just pocket money?