Limitations

Technically you can get multiple of the same sku back to back and be fine, but if you do this too often you can risk getting banned. This also doesn't have to be only with the same sku, I remember there was a huge dunk restock and I got 4 different SKU's within 2mins and I got banned.

So a rule I follow now is to wait until the first item ships before you go for another, just don't be too greedy.