idclient | month_transac | sales
 ----------+---------------+--------
 511656A75 | 2010-06-01    |  68.57
 511656A75 | 2010-07-01    |  88.63
 511656A75 | 2010-08-01    |  94.91
 511656A75 | 2010-09-01    |  70.66
 511656A75 | 2010-10-01    |  28.84
 [...]
 511656A75 | 2015-10-01    |  85.00
 511656A75 | 2015-12-01    | 114.42
 511656A75 | 2016-01-01    | 137.08
 511656A75 | 2016-03-01    | 172.92
 511656A75 | 2016-04-01    | 125.00
 511656A75 | 2016-05-01    | 127.08
 511656A75 | 2016-06-01    | 104.17
 511656A75 | 2016-07-01    |  98.22
 511656A75 | 2016-08-01    |  37.08
 511656A75 | 2016-10-01    | 108.33
 511656A75 | 2016-11-01    | 104.17
 511656A75 | 2017-01-01    | 201.67

SELECT t1.idclient
    , t1.month_transac::date
    , t1.sales
    , SUM(t2.sales) as sales_ttm
FROM temp_sales_sample_month_aggr t1
LEFT JOIN  temp_sales_sample_month_aggr t2 USING (idclient)
    WHERE
        t1.idclient = '511656A75' -- for example only
        AND t2.month_transac >= (t1.month_transac - interval '12 months')
        AND t2.month_transac < t1.month_transac
GROUP BY 1, 2, 3
ORDER BY 2
;

idclient  | month_transac | sales  | sales_ttm
-----------+---------------+--------+---------
 511656A75 | 2010-07-01    |  88.63 |   68.57
 511656A75 | 2010-08-01    |  94.91 |  157.20
 511656A75 | 2010-09-01    |  70.66 |  252.11
 511656A75 | 2010-10-01    |  28.84 |  322.77
 511656A75 | 2010-11-01    | 110.38 |  351.61
 511656A75 | 2010-12-01    | 125.67 |  461.99
 511656A75 | 2011-01-01    | 108.45 |  587.66
 511656A75 | 2011-02-01    |  83.21 |  696.11
 511656A75 | 2011-03-01    | 102.73 |  779.32
 511656A75 | 2011-04-01    | 254.09 |  882.05
[...]

idclient  | month_transac | sales  | sales_ttm
-----------+---------------+--------+---------
 511656A75 | 2010-06-01    |  68.57 |    0.00
 511656A75 | 2010-07-01    |  88.63 |   68.57
 511656A75 | 2010-08-01    |  94.91 |  157.20
 511656A75 | 2010-09-01    |  70.66 |  252.11
[...]