The Black Diamond came with one of two barrels, 22.5" or 26" long; the 26" barrel was called the XR model. That's probably the Express you're looking at. There are also three different ignition system options: standard No. 11 percussion cap, musket percussion cap and 209 shotshell primer.
The Fifth Edition (2007) of the Blue Book of Modern Black Powder Arms gives the value of a 100% condition Black Diamond XR as $300; in 80% condition it's $115.
You can find a manual and marketing information on the Thompson Center web site; just search for Black Diamond.
Compared with sidelock (flint or percussion) rifles, inline black powder rifles have the advantage of a 'closed' ignition system, in that once the bolt is closed the cap or primer is protected from the elements while still in a ready state for firing. Most of them are also 'rated' by the manufacturer for very heavy powder loads, on the order of 150 grains (whether this is actually an advantage is debatable, however), as opposed to 100 or 120 grains for sidelock guns.
With respect to cartridge rifles, inlines have the advantage in some states of being usable during the special 'muzzleloader' or primitive deer hunting seasons. This is not true in all locations, however, so check with your local authorities about whether your particular weapon is legal for that season. Finally, the addition of a scope is generally easier if desired; some come with scope mounting provisions already incorporated (I don't believe this is true with the Black Diamond from the factory, but they may have been added later).
There are two significant disadvantages, in my opinion. One is the bolt is a mechanical device with close tolerances and thus subject to problems with fouling, and black powder shooting is famous for cap and powder byproduct fouling. While not necessarily prone to fouling, it is a good idea to pay special attention to keeping the bolt assembly clean.
Second, and more important to many people, an inline rifle is not considered an 'authentic' weapon. Guns with percussion ignition systems are throwbacks to an historical time that many feel was a better lifestyle than we have today. Inline guns are a modern application many believe were invented just to get around the restrictions of the special muzzleloading hunting seasons and extend their hunting opportunities. They believe the 'primitive' season regulations should be more literally interpreted to prohibit the use of inline rifles, and many states have modified their regulations to make that so. So, the inline rifle has not gained social acceptance in many areas.
With respect to what to look for, condition is more important for a black powder gun than a smokeless powder gun simply because the combustion byproducts of black powder are highly corrosive. Debilitating damage due to corrosion is more common and easier to experience in black powder. So, the most important thing is to thoroughly and carefully inspect the barrel bore with a bore light for rust and pitting prior to purchasing the gun. If it's dirty, walk away. It may or may not be in pristine shape under the dirt, but if you can't see it, you must assume there are rust pits. Also, remove the barrel and inspect the action parts for rust and especially dirt/crud. While there is less likely to be corrosion damage, the cleanliness of the action is an indication of how well the gun was treated. Again, lots of dirt, hit the road.
You indicated you are new to black powder. The Black Diamond is a good starting point. It will require you to learn the art of black powder shooting - each and every load is built from the ground up, and each and every gun needs to be tested to find the load it 'likes' best. Inline guns tend to be more forgiving of minor variations in loading technique than the general body if sidelocks, but they still exhibit the black powder characteristic of having a 'sweet spot' combination of powder, projectile, lube and loading technique that is better than any other. Finding that combination is really the fun part of black powder shooting.
There are several tutorials and books available to guide you through that. I suggest reading through the offerings in the Black Powder Essentials thread at the top of this forum.
