From a European legal viewpoint on replica's, using a 550 as example.
A car brand con only take action against replicas in case of :
1. Trademark infringement : the use of name tags (Porsche, Spyder), logos (the Porsche shield), on replicas. TM registration can be renewed indefinitely. Porsche, Spyder and porscheshield are registered and still protected. (but see info below)
However, the form/look of a car is inherent to 2. Design registration registered for 5 years with option to be renewed 4 x 5 years, for a total of 25 years.
3. Copyright : is only applicable to works of arts (books, music, paintings, sculptures) We may consider a 550 as a work of art, but it is legally "just" a car.
4. Patent : technology innovations are protected under that system and can not be copied. As long as you don't copy Porsche technology and put it onto a replica, you're safe.
In short : a replica owner can sell his replica car of brand X (Beck, etc..) as such, as long as he doesn't sell it as a Porsche. No law forbids a replica owner to mention that the replica is a tribute to an iconic Porsche model and is not an exact copy. Legally a replica owner could even put Porsche and Spyder emblems and Porsche shield on his car, BUT ..... in this case I would strongly advise you purchase them from Porsche directly. Using authentic Porsche purchased material, you are not infringing TM on a replica because Porsche is not loosing money out of the use of their TM names, as you purchased it from them : you have just put authentic Porsche material on a Porsche replica.
However, when you sell the car, do not tempt Porsche : take away all Porsche material and replace back the original badges etc... and advertise it as a Brand X replica of Porsche.
As to professionals in this market : Here another aspect comes into action : the intend to make profit !!! Not a single replica builder sells their replicas with Porsche emblems or names on it, but they do intent to make profit out of the fact that their car looks like a Porsche. But then.... if the car has no valid Design registration (point 2 above) then they are not breaking the law.
All this in theory, because if Porsche sees an add anywhere of a replica on a website, and contact the website, I'm sure the website will not even argue with Porsche and just delete the post, unless the seller has also mentioned to the website in question that they would take legal action against the website for removing the ad without legal ground.
In our regional Porsche club, managed by the local PC, we have 1 guy coming to events with his Chamonix (Brasil) 550 replica with Porsche emblems etc... He's a member just like all of us !!!
Personally I think Porsche has bigger issues that 356/550 replicas : http://carnewschina.com/2016/10/26/how-much-exactly-is-the-zotye-sr9-a-clone-of-the-porsche-macan/http://carnewschina.com/2016/10/26/how-much-exactly-is-the-zotye-sr9-a-clone-of-the-porsche-macan/
well, just wanted to add my 2 cents worth :-)
Drive happy and safely !