Parts..... they are very desirable from Black Edition A3/S3's alike! Take a look round the forum here for instance, there is usually many threads about people retro fitting S-Line bumpers, steering wheels, interiors, and wheels etc.
You're absolutely right.
For fear of offending anyone (as last time I raised this one of our members thought I was having a dig), there is a rampant market for upgrades, and if you spend any time on e-bay looking at A3/S3/RS3 you'll see quite a list of cars that have been made up to look like something else (only last week someone posted about an A3 1.6 RS3 lookalike).
I hold two reasons for this view:
1) If people bought the parts over the counter at Audi and then did the upgrades it would be non economic to do this with the sole intention of selling the car straight on afterwards - you'd never make your money back
2) Half of these cars that turn up for sale never seem to be part of anybodies build threads on any Audi forums I visit. If the people doing it were passionate they'd be doing build threads. I know I'm generalising a bit as not everyone will use internet forums, but there seem to be a steady flow of these cars that you never see sight or sound of on here
Unless the parts are being acquired by 'dubious' means, it doesn't make for a profitable business model to be doing it, so it makes you wonder if there is a cottage industry just replicating high spec cars in lower rent donor shells being fed by crime. Also the parts in isolation on ebay (like wing mirrors) make good money.
These days I keep a very close eye on threads and questions being asked in the 8P section to see if there are any patterns or suspicious questions being raised that would cause a need for any concern.
Obviously as you're more than aware, do take every precaution, implement what safety measures you can, and don't think just because your car isn't an RS that you're immune, your not. Given the number of Leeds threads we get, if they were to catch this group it makes you wonder whether it would balance things out, or whether others would just step in to take their place. Tragic.