Whilst I like to grab a bargain, where I can, I don't begrudge anyone from selling stuff on for as much as the market will allow.
There is a bit of a difference between advertising a 'desirable' item for a high-ish price, and holding a part to ransom if that bit is needed to get a car back on the road..... but that is maybe ethics, not economics, and my view is irrelevant to that of a parts-seller.
A set of wheels at £500 would seem okay to me..... a set at £2000 isn't! - - - - but that is easy to say when I don't need any. If my car was involved in an incident where it needed a couple of wheels, and maybe a few other bits, I can fix it with parts (at a cost) and keep it..... others may end up with a write-off..... but if a car is about to get written-off, maybe £2k for a set of wheels seems a bit of a bargain?
As for the aerial, well, the seller has a rare item, and it is up to the prospective buyer to decide if they want to pay that much, or if they wish to seek and fit an alternative. Neither answer will keep the car off the road, and TBH, there are plenty of options out there other than the OE aerial.. unless you really really want to keep it original.
To some extent, the same argument applies to the wheels - the bolt pattern is 'standard' 4-stud Vauxhall/VW so there are plenty of others that will fit, in various sizes from 14" to 18" diameter (okay, 14" steels will not be great, but they will fit) - in fact, some of the Corsa D wheels look more than okay on an Elan (16" & 17").... but you will have to accept that the handling may change a bit.
As for the ECU's - well, that may be a bit different. You NEED one of those. If yours goes wrong, you NEED to get another one - and they are sort-off specific to the car. ie, the MEMCAL will be specific to your own cars' set-up..... but the rest of the box isn't! (unless you have an NA - they are actually a different unit).
As such, ANY ECU for an SE/S2/LHD/RHD/FED/ROW car will do the same thing, as long as you fit the MEMCAL that you require (either from your old unit or elsewhere). So buying an ECU for a UK SE for £xx may be better than buying an ECU for a FED car for $yy.... or vice versa.
And that is before you even consider replacing the whole gubbins with an aftermarket ECU (which can be done for far less than the advertised prices of some ECUs these days!). Even with a couple of hours on a rolling road to set it up, a Megasquirt system could be bought, fitted and tuned for less than £500...... I use Megasquirt on another car and it can even be programmed whilst you drive it (with a second operator - obviously!).
This is a bit

for this thread, but I suppose it is relevant really. Would I want to pay £xxx for a rubber seal? - probably not - but that is because I don't need one!
My car lives in a garage, and comes out when it is dry (or if it rains whilst I'm out, so be it) with the hood down..... and my seals are reasonably okay anyway, for the odd time it is parked outside with the top up, in the rain.
But if the car lived outside and leaked, I would see the price as a real bargain!
Having been through the pain or repairing my own outriggers due to rust a fair few years ago (and the car has been garaged ever since), and having the privilege of doing the same for someone elses car last weekend, I know the value of keeping the interior dry (and it isn't just to do with driver comfort) - so these bespoke items then seem cheap-as-chips.
Besides which, Jamie has never made a secret of the process to make them yourself - so there is nowt stopping anyone with the time and patience from doing similar.
Having said all that, there are some second-hand parts sellers who do things I disagree with, and I won't give them my custom if there is an alternative.... but there isn't always an alternative..... so I have to suck it up and pay the ransom prices occasionally (except in reality, I'll leave the car in the garage and wait until I can fix it at a reasonable cost - I have that luxury).