As
BobK rightly says you need to have a browse through the stickied threads at the top of the board.
It's not entirely clear from your question what you're actually after.
There are certain "Special Cars" which cannot be raced in GT Mode.
There is a link to that list here (which you'd have found if you'd looked in the "LOOK HERE
BEFORE YOU ASK - MANY FAQs" thread.)
However, only 2 of the cars on that list: Nike One 2022 & Toyota Motor Triathlon Race Car 2004 are concepts.
All the other "concept" cars in the game, most of which have since gone into production after the game was released are raceable immediately, but have no resale value. You can "sell" the car in the usual manner, but since it has no value the game asks if you want to give the car away?
Examples of this include:
Audi LeMans Quattro Concept '03 - Released as Audi R8
Audi Pikes Peak Quattro Concept '03 - Released as Audi Q7
Nissan GT-R Concept (TokyoShow) '01 - Released as Nissan GT-R
Mazda RX-8 Concept (Type-II) '01 - Released as Mazda RX-8
Pontiac Solstice Coupe Concept '02 - Released as Pontiac Solstice Coupe
Infiniti FX45 Concept '02 - Released as Infiniti FX45
Land Rover Range Stormer Concept '04 - Released as Land Rover Range Rover Sport HSE
Mazda MX-Crossport Concept '05 - Released as Mazda CX-7
etc.
All the other concepts which didn't make production, such as:
Mazda Kusabi Concept '03
Mitsubishi i Concept '03
Suzuki GSX-R/4 Concept '01
Toyota RSC Concept '01
VW W12 Nardo Concept '01
Hyundai HCD6 Concept '01
Acura HSC Concept '04
Audi Nuvolari Quattro Concept '03
etc.
are also can also be raced immediately, however, there may be certain restrictions per race event which may not allow concept cars to be raced, and these are generally highlighted in the race requirements as "Production Cars Only". There's some known glitches with this, such as the Supercar Event in Professional Hall where you cannot enter a VW W12 Nardo Concept '01 or Cadillac Cien Concept '02, but the AI, somehow, is allowed to drive them.
Suggest you take a look at the attached guide. It'll tell you when & where to win everything, what the race & tyre requirements are, how much prizemoney you'll obtain, and has a complete list of all the cars in the game (some duplicated names for PAL / NTSC game.)
This is also a great place to look for stuff:
Digital-Nitrate Lists/Info Links
Oh, and this is the 2nd time I've had to post this warning to you. Continue to ignore it and I'll have one of the active staff members make it an official warning.
You need to make a little effort with your posting, it's a requirement of the Acceptable Use Policy that you agreed to when you registered that:
You've used "textspeak" twice in that post alone, not to mention begun three sentences without capital letters. Bring your posting quality up to scratch please if you want to continue posting here.