Exactly as Alan Merklin said (and see my previous post on the subject) : legally it is all about "shape" and "brand" :
Porsche can protect and register it's brand name and logo's indefinitely under the TM laws, but the shape of a 356/550 etc... falls under the visual design patents, and these are only protected for max. 25 years, but you need to add to this the aspect of "intend to make profit of the name of Porsche", as explained below :
John Doe selling his 356/550 replica stating "a Brand X replica/tribute/evocation/remembrance etc... of the famous/iconic P356/P550" does NOT brake any TM/design laws (Porsche lawyers, if you're reading this.... you already have my email as we communicated before), as long as you do not sell it as a P356/550 car. Now, when a trader buys/sells a 356/550 or whatever other replica.... the story is different, because a trader may do this on a regular base with the INTEND to make profit on the fact that the car in question LOOKS like a 6M $ 550, even if he states it's a replica (he will never used the word "copy")
Why did Porsche never sue Beck (USA), Chamonix (Brasil), RCH (Greece) and so many other replica builders in the USA or UK ? Because NONE of these companies ever advertised a 550 replicas with Porsche names/logos/shields. They advertised these cars with their own brand name, just stating that these cars were a tribute (or whatever word you want to use for this) to the historic 550 car. The fact that the buyer afterwards add name and logo to it doesn't change anything to the legal argument : Buy a "Trabant" (the former east-german car) and put Porsche brand marks on it (even original) : NEVER will Porsche sue you, even if you use chinese knock-off logos, because it's not worth it (for the logo's), but use a non-protected shape of Porsche, and their lawyers will send you a mean letter.... but never did this go to any court !!!
And yes, eBay will perhaps take down ads, not because there is a legal ground for this and to protect eBay, but just because when a company gets an email from Lawfirm X (I will not mention them here) in Germany, it is logical that the company in question will not start a legal arguement between themselves and P-lawyers, just to satisfy their customers. The RISK for eBay to advertise P-replicas is greater than the REWARD they get to advertise a P-Replica, because once eBay lawyers needs to respond legal arguments to P-lawyers in defend of eBay customers, who will pay that bill ? eBay !!!, not the guy/trader selling his P-Replica on eBay.
Want to take on a challenge ? how many 356/550 replica owners worldwide are there ? No idea... but even if only a fraction of them (let's say 2.000) put 100 USD/EUR aside on a crowdfunding, we have 200.000 USD/EUR to defend the first guy who says to Porsche lawyers : "OK, bring it on... let's go to court for having a replica, not protected by design laws !" Even pure Porsche owners (997.2) like myself will say "if the original 550 model is so expensive now, why doesn't P bring out a same-shape but new technology model 550 out on the market then, instead of suing guys who put a lot of money (40K at least) into a car build like the original 550, which was in its day a fantastic race but today is simply a fun car which may be the car in which you will be killed, because after all ... even the original one is a death trap in case of a serious accident involving any other modern car ? Just imagine the negative publicity ...
The answer is simple : For P that build a modern 550 car, it would result in a market price close to the 917/Boxter but still as unsafe as a original 550 or a current replica.
Owners of "real" Porsche love P-cars for the history and evolution of the P-cars, and some of them have a 356/550 replica as 2nd car, not to own 2 Porsches, but to have the pleasure and sensation of driving a race car build like it was build in the 50's. I respect such replica owners (even if they only own a P-replica) more than the owner of a real 550 and keeps it as a garage queen, or brings it to a show on a trailer.
I once met a guy who had an original 550 in Europe (and the car does many appearances in EU) : the car is in perfect immaculate condition and does about 1.000 miles / 600 km per year (short rallys or shows - getting there on a trailer).
I told him : "You are so lucky to own an original 550, it's one of the most beautiful cars ever build but you barely drive it once every 2 or 3 months... (and I apologize in advance if the following sounds sexist to any reader), and to me this is like being married to Miss Universe and only spend a nice evening with her on a quarterly base just to look into her eyes ! " What I mean is : If I own/(am married to) a original 550/ (Miss Universe), I will enjoy/(enjoy) driving/(being married to) my 550/(my beautiful Miss Universe) as much as I can/(every day).
Again, before we start a gender/sexist war here .... ladies .... If you are lucky enough to have Brad Pitt/George Clooney/Christian Bale or any other star as a husband, would you just "enjoy his company" once every 3 months because there's only a few of these guys in the world and they're worth millions ? No..... :-) see what I mean ? Same with cars !
OK, I started very legally, then moved onto more funny aspects, but at the end of the story it's very simple : we all like driving classic/oldtimer cars build as they were in the days. Very few of us are lucky enough to have several hundreds to millions of dollars to buy a real one, so let us just enjoy driving our cars, originals or replicas in the same clubs (Porsche... are you getting this ?)
Drive safely... in a real one or replica !