how much can i sell ectos for to the vendor?

i see the selling price atm is 7.5 k, so how much can i get for about 140 of them?

The easiest way to find out is to get a quote from the Rare Material Trader. Put the 140 ectos in your inventory and talk to the trader. Just like when you buy something, he will give you a quote if you try to sell them, which you can Cancel without actually selling. He will give you a quote per ecto, so you'll have to do the math yourself (but, of course, there is a calculator in Windows).
If the current sell price from the trader is 7.5k, the buy price is probably 6.5k or so = 910K or so for 140

P.s - use the Rare Material Trader, not the Merchant.

P.p.s. The usual price that other players will buy your ectos at, is half way between the Trader's buy/sell prices - in this case ~ 7k. So you would get the most gold by selling to other players (in Kamadan, etc). (140 x 7k = 980k)

Also keep in mind that if you're selling them in order to buy items, it may be easier to use the ectos as currency. For example, if you want to buy an item worth 20,000g, you can offer 3 ectos instead. This is esepcially important in the high-value market.

The price may very well decrease as well. You may not get the same price for the last ecto as you do the first.