[andrewSF]

Oops, Phil beat me to the short answer so here's a longer one:

"Native" can mean any of a number of plug-in formats that run on a host computer's processor. The native format for Pro Tools LE is RTAS (HTDM is also native, but is not for LE). Other common native formats include VST and AU (Audio Units). The non-native format for Pro Tools is TDM and the plugs require specialized hardware (the PCI cards) to run. To make it more confusing some plug-ins run on specialized hardware (UAD-1, SSL Duende) but appear in Pro Tools as RTAS plug-ins, IIRC.

Some plug-ins are available in multiple formats and the price may vary according to format. If you have a VST to RTAS wrapper you can run VST plug-ins, but generally you'll want to make sure that anything you buy is RTAS.

If you're buying used you should be sure that the version you are buying is compatible with the version of Pro Tools you're running. Check the Digidesign website and the plug-in manufacturer's website and ask for all of the details from the seller.

While you're looking at the manufacturer's website, make sure that you can transfer the license for the plug-in. If you can't transfer the license, you won't be able to get any support or upgrades from the manufacturer. With iLok this is usually a pretty painless process.

Also it's wise to check out the copy-protection scheme that the manufacturer of the plug-in uses. If, for example, it uses iLok and the person you are buying from says something like "you won't need iLok for this version" I would be quite sceptical about whether it is legitimate. If the plug uses a challenge-response, be absolutely sure that the license is transferrable or you won't be able to authorize the plug-in on your computer. Again, if the seller suggests that you won't need to worry about authorization, look elsewhere.