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Buyers & sellers in ECNs are traders, or investors. I'm not quite sure why the FR uses "émetteurs de liquidités"; liquidity is a property or characteristic of financial products, assets and markets, not something that can be traded or issued.

Wikipedia: An electronic communication network (ECN) is a financial term for a type of computer system that facilitates trading of financial products outside of stock exchanges. The primary products that are traded on ECNs are stocks and currencies.

Or possibly, though less elegant, "issuers of liquidity"

liquidités (COMMERCE) liquid assets
http://dictionary.reverso.net/french-english/liquidités

liquidités (FINANCE) liquid assets
liquidités internationales international liquidity
http://www.larousse.com/en/dictionaries/french-english/liqui...



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Note added at 2 days29 mins (2012-11-25 12:48:48 GMT)
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Not to put too fine a point on it:

liquidité(2)
[Correspond à liquide(2)] ,,Aptitude d'un bien à se transformer plus ou moins rapidement en monnaie`` (Gestion fin. 1979).
− Au plur. Somme disponible immédiatement.

liquid assets
assets in the form of cash (or easily convertible into cash)
http://dictionary.reference.com/browse/liquid assets?s=t

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Note added at 2 days46 mins (2012-11-25 13:05:34 GMT)
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And from the GDT:

émetteur
Domaines
droit
finance > valeurs mobilières
Définition Personne ou organisme qui émet des effets de commerce (billets, chèques, etc.) et, dans le cas de titres (actions ou obligations), société qui offre ces titres à des investisseurs.
Anglais
Terme issuer
http://www.gdt.oqlf.gouv.qc.ca/ficheOqlf.aspx?Id_Fiche=50293...