Вот сам запрос без ограничения на количество товаров:
SELECT el.ID, el.NAME, el.DETAIL_PICTURE, price.PRICE, prod.QUANTITY
FROM b_catalog_product AS prod, b_catalog_price AS price, b_iblock_element AS el
WHERE el.IBLOCK_ID = "6" AND
el.ACTIVE = "Y" AND
prod.QUANTITY > "1" AND
price.PRICE >= '. $_POST['MINPRICE'] .' AND price.PRICE <= ' . $_POST['MAXPRICE'] . ' AND
el.ID = prod.ID AND
el.ID = price.ID AND
prod.ID = price.ID
ORDER BY RAND()