No idea about any technical e-jargon for this, but could you not simply use "sites popping up like mushrooms" or "mushrooming"? (For your first quote anyway; not really snappy enough for the second quote you give - just "ephemeral sites"?)

This can convey the idea but is a little longer...

http://www.screensaversgroup.org/tag/mushrooms
http://www.gerrymcgovern.com/nt/2004/nt_2004_09_20_universit...

Very few references but the idea is put across clearly and there can be no confusion.

Or, in Internet-speak, e-femural sites.

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Note added at 13 mins (2010-04-23 07:29:41 GMT)
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Also "fly-by-night sites".

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Note added at 16 mins (2010-04-23 07:33:36 GMT)
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Google™, Yahoo™ and others are engines that search the internet looking for sites, ... This keeps the TEMPORARY OR FLY-BY-NIGHT SITES out of the list, ...
www.industrywebhost.net/.../1.--Enabling-new-clients-to-fin...

One word of caution here, beware of FLY-BY-NIGHT SITES, WHERE YOUR RISK OF GETTING COUNTERFEIT ... While the cost of accessing the internet keeps rising, stick it to the man, ..... Cardiovascular specialists and care provider glossary ...
weblogs.fox40.com/6degrees/2009/07/

Sounds like he's talking about the FLY-BY-NIGHT SITES THAT CLAIM TO SELL ... on the Internet for people who deliver no value whatsoever.” ...
www.avsforum.com/avs-vb/showthread.php?t=1105773&page...

With more than twice the global penetration of the Internet, ...... Scams, pyramid schemes, dodgy traders and FLY-BY-NIGHT SITES ABOUND: none of these are ...
mediachannel.org/keracarehairproducts1/

1. by MP Persellin - 2001 - Cited by 4 - Related articles
Similarly, the advent of the Internet has virtually eliminated ...... The Hazards of Buying Drugs Online: How some FLY-BY-NIGHT SITES RAISE THE RISK to ...
https://litigation-essentials.lexisnexis.com/.../app?...1...

Beware of the scammers and FLY BY NIGHT SITES THAT HAVE POPPED UP all over the internet in these tough economic times. ...
ezinearticles.com/?Is-White-Gold-More-Precious-Than...


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Note added at 1 hr (2010-04-23 08:34:29 GMT)
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You yourself said they often sell counterfeit goods, which is mentioned in at least one of the refs above. Another uses "temporary", another "popped up" (like mushrooms).
In general they are disreputable.

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Note added at 1 hr (2010-04-23 08:38:47 GMT)
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Moreover, "fly" carries notions of "flight" and "flee", hence "fleeting" (momentary), and Webster's says of "fly-by-night "not trustworthy; esp., financially irresponsible. - n. a fly-by-night person; esp. a debtor who runs away to escape debt"

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Note added at 1 hr (2010-04-23 08:55:24 GMT)
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Every website needs a FAQ page so you don't look like JUST ANOTHER "FLY-BY-NIGHT" WEBSITE THAT WON'T BE IN BUSINESS FOR LONGER THAN A MONTH OR TWO. ...
www.templatetimes.com/scripts.php

Then there's "flash-in-the-pan website". This certainly means shortlived ("e-femural", or "ephemeral" if you must), but this may be due to the phenomenon of fads and passing fashion, not to the fact that it's dishonest, disreputable, or fraudulent, which are associations your text seems to evoke. However, this DOES sit well with your surfer plus ou moins bien sur la tendance de la sortie amicale.

Google tends to reward content much more so than a FLASH-IN-THE-PAN WEBSITE THAT DOESN'T OFFER ANYTHING OF VALUE. The only thing to do therefore, ...
www.article-bank.com/.../Sifting-Through-the-Google-Sandbox... -

GIO is a well-recognized and trusted brand, unlike the competition, not a FLASH IN THE PAN WEBSITE THAT YOUR CUSTOMERS WON'T KNOW, trust or buy from. ...
www.giobikes.com/Wholesale-Program/

building a PROPER BUSINESS HERE, NOT A 'FLASH IN THE PAN' WEBSITE. ...
paulteague.com/2009/03/07/month-4-in-the-sbi-house/

1. If you are looking to create a FLASH-IN-THE-PAN WEBSITE, along with the management of very angry visitors infected with malware, then free web hosting may ...
www.most-best-host-test.com/web-hosting-articles/Free-Web-H...

WILL BING BE A FLASH-IN-THE-PAN WEBSITE? It launched about a month ago and is already ranked the 39th most popular site on the web. ...
www.tblc.org/newsletters/July2009/featurecontent.html -

We8there.com is NOT A FLASH IN THE PAN WEBSITE, WE HERE TO STAY. It's because of our faithful users, that we have remained a strong presence. ...
www.facebook.com/topic.php?uid=47670742650&topic=9327



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Note added at 1 hr (2010-04-23 09:02:33 GMT)
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If champignon in your text covers both "fly-by-night" and "flash-in-the-pan", one translation trick is to use one of these expressions on first encounter (the one more immediately relevant) then move on to "These xxx (the other term) sites ..." where "these" obviously refers back to the first encounter.

Alternatively, use both, together. Your 3-item list Sites champignons, plateformes d'enchères en ligne, e-commerçants for example would simply become a 4-item list.

If you go for the last options, will I get double points?

1 a : to well up and spread out laterally from a central source b : to become enlarged or extended : grow.

A different interpretation.

That are taking advantage of more "friendly" sites