One problem with buying elsewhere is many of them aren’t “plug and play” with a different engine. For something like a mesh or audio that’s not a big deal, but pretty much everything else it will be. Some assets will require some work on your part to convert over, so you have to take that into account too. Even for a mesh asset there could be Unity style collision which might not always work well in UE4, the material might be a custom shader that would need to be duplicated, etc.
So part of the answer is asking yourself whether a small price difference is worth it vs the time you will spend converting it? That can’t really be answered by any of us.
Not saying this because I appreciate the price difference by any means, I don’t think it’s right at all and I hope the prices become more competitive, but there is more to consider than just the price when purchasing assets elsewhere.