SELECT 
  cscart_products.*, 
  cscart_products.ebay_status, 
  cscart_products.ebay_template_id 
FROM 
  cscart_products 
WHERE 
  cscart_products.product_id IN (
    21519, 21520, 21521, 21522, 21523, 21524, 
    21525, 21526, 21527, 21528, 21529, 
    21530, 21531, 21532, 21533, 21534
  )

Query time 0.00043

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_products.product_id in (21519,21520,21521,21522,21523,21524,21525,21526,21527,21528,21529,21530,21531,21532,21533,21534)"
    }
  }
}