The biggest differences between 9(HD) and 9 with the CPTK are TDM processing, input monitoring, system latency, and reliable track count.
TDM processing is great for mixing, but you will obviously pay more for it. You will also pay more for plug-ins that are designed to work with the TDM chips versus what you would for RTAS/Native plug-ins. Input monitoring is useful, but not game changing (bigger in the music community than in the post/design community). System latency is system latency. It goes (from lowest to highest): TDM HD, HD Native, Native. As for track count, this is going to be dictated by your system. Even TDM HD systems have track counts that are limited by the number of TDM cards you have.
To put this in perspective, I have a session that I've run at both home (PT9+CPTK) and work (TDM HD3 system), and neither one can run the full track count. I've had plans to break it out for more detailed pre-mixes anyways, but it was an interesting test.
As far as eBay purchases go, you have to be EXTREMELY careful. Most of the HD systems that you are seeing up for auction are being resold by someone (rarely are they the original owner). The problem is this...if they aren't the original owner, they may not have all the necessary data and authorization to complete the "change of ownership" process. If you can't get that done, you're going to have a very hard time getting your system registered with Avid. If you can't get it registered with Avid, you will not be able to upgrade the software unless you end up buying yet another used license down the road...repeating any time you want to upgrade.
If you decide to purchase a used LE system for the upgrade path (as Steve did), make sure that the owner hasn't already used it themselves to upgrade to 9. There are a number of people who have upgraded using the serial number on their hardware, then sold the hardware because it was no longer required. Only 1 upgrade to 9 is allowed per hardware unit. Meaning you could be in a similar situation to what I described with the HD systems.
Summation...9 with CPTK is pretty powerful, and you're going to have to be involved in some very serious work to notive a measurable difference between 9 and 9HD...and make sure you're educated and ask the right questions before buying ANY used Pro Tools systems/licenses (be they 9 or 9HD).