I think the real question is: "What is a reputable seller?"
Personally I look at product warranty, return policies and costs and what international standards are referenced. Many international sellers are counting on the fact that return shipping will cost as much or more than a product is worth so ..... even unhappy customers won't return anything.

For example - the last batteries I bought were from a seller within the country and came with a two year warranty - in spite of being an item on clearance. If there's an issue they'll be replaced with an equivalent or upgraded model.