1.There's a 45 day (from shipment date) guarantee that they get a refund if an item gets lost in the mail
2. They can also request a refund if the item is not as described or is damaged.
3. They can request a refund for any reason that they are not happy.
YOU decide whether a refund is warranted. If so ... give them one straight away ... then you contact the Aliexpress seller to give you a refund. In the unlikely event they don't give you one ... too bad. That's life. That's business. Your overall profits will absorb the cost. Don't list crappy products from crappy sellers.
Returns ... Not 100% sure. I've never had to do one.
As for returns, going by my own experience from ordering stuff for myself, 9 times out of 10 the Aliexpress seller will let the customer keep the item. In the rare case when they want it returned, you will have to get an address (anonymous) from the seller and ask your customer to return it to that address ...
I THINK ... anyone else got a definite answer?
Because of the above ... I stay right away from really expensive items and anything that has sizes ... ESPECIALLY SHOES! You really have to go to a brick and mortar store and try shoes on. I have ordered shoes for myself 3 times and none of them fit properly. Same with jackets and shirts. But the fact that every man and his grandmother are selling clothing and shoes is just another reason I stay away from them. Not an original enough niche.
And finally ... read this for the actual process ...
https://alidropship.com/codex/returns-and-refunds/
Then read ALL of the Knowledge Base and Blogs ... lots of of handy information there.