READING WILL SAVE YOU MONEY AND TIME!!
The big problem is most people DO KNOW that they have to have the right mhz remote/part number to work/program into their vehicles locking system.
Most DON'T KNOW that the transponder in the lower half of all VW keys is the most important part, and not all are compatible with each other.
The transponder is the small chip in the key which communicates with your cars immobilizer system, allowing the engine to be started in the first place.
IF THE TRANSPONDER IS FROM A DIFFERENT MANUFACTURING BATCH/YEAR TO WHEN YOUR CAR WAS MADE IT WILL NOT WORK/PROGRAM INTO YOUR CAR, AND YOU WILL BE LEFT WITH THE £35+VAT PROGRAMMING FEE AT THE DEALERS, (IT'S CALLED VIN SPECIFIC).
I bought two keys one of which was brand new from ebay, and confirmed with both sellers it was compatible, AND TWICE IT WASN'T!
Having spoke to the engineer on my car at VW he confirmed that the batches used in different years of manufacture were not compatible with each other, hence not programmable.
My advice is, if you want a replacement remote (£33+vat from the dealer) DON'T OVERPAY thinking they are expensive, and you could get a bargain, but if you want a full key including transponder (it will cost you £150 from the dealer fully cut and working) don't be fooled into thinking these used keys will save you a fortune, they won't, unless the seller can verify what model and year the key came from and this matches your car, the likelihood is it won't be compatible, and you'll have wasted your money.
Getting a key cut is a locksmith's job for models with the HAA blade, this is £10-20 depending who you call, as not all will do them, coupled with the £41.12 programming fee it will cost you over £50 before you've bought the key...EVEN IF IT DOESN'T WORK !
Some of the more clever listings offer "guaranteed money back" if it doesn't work, what they don't tell you is that you have to return it in an unused condition, which is impossible when you have to cut the key to program it (as it needs to be in the cars ignition to be programmed), and no refund on postage either (which is usually inflated) so they know their money is safe once they've got it.
Look out for the brand new lower half keys, most are FAKE copies, these are just as troublesome, ask if it is a genuine part before buying, you can tell as soon as you touch one, they feel cheap, the blade doesn't fold right inside, and they don't fit the remote flush either, and whether you can have a FULL refund if it turns out to be knock off! (see pictures at bottom of guide for reference)
Feedback can also be misleading, as most will leave it when they receive their key, not after they realise programming isn't going to work.
I also learned that independent locksmith's will not touch used keys from ebay, or anywhere else, as they are very rarely compatible, and take longer to attempt to pair due to them failing to program the first time, they have to try twice, and then customers don't want to pay because they don't work at all, I would advise ringing a locksmith before buying and asking them, you'll see what I mean, I couldn't find one that would do the job without buying a new key from them!
Ask yourself, is it worth the risk? as it could end up costing you £150 anyway, as well as the money you have wasted on one that doesn't work.