The set in question is designated LLCA53 Han Solo On Tauntaun in the Brickset database, and it's apparently owned by just 14 of Brickset's 117,000+ registered members. The previous owner was given the set by LEGOLAND California as a thank-you for contributing Lego Star Wars photographs to their Fan Gallery, although it appears that the sets were originally designed for Ambassador Pass holders attending the 2011 Master Model Builder Class. The set wasn't supplied boxed - the elements are divided between three numbered grip-seal bags which are themselves packed into an opaque LEGO Factory bag along with six folded and loosely stapled sheets containing the building instructions. The instruction sheets are colour printed on both sides; the artwork above can be found at the top of the first page while the inventory of parts below (click to enlarge) is printed on the back of the last page. The quality of the printing isn't up to retail standards from the perspective of either sharpness or colour representation, which isn't altogether surprising in a promo set such as this.
Bag 1 contains the parts needed to build the display stand, which is of course designed to look like snowy terrain, and the tauntaun's feet. This stage of the build utilises common elements - only the six tan modified 1 x 1 plates with tooth which form the claws on the tauntaun's feet could be described as remotely uncommon, and even these have appeared in 20 sets to date.
Bag 2 contains the parts needed to build most of the tauntaun plus its harness. Once again, the parts used appear to be unremarkable - predominantly light bley slopes of various types with a few common plates, small bricks and clips thrown in for good measure. Certainly I don't think sourcing them would be too much of an issue for would-be builders. A number of SNOT bricks, specifically 1 x 1 bricks with a stud on one side, are utilised to hold the harness in the correct alignment and provide an attachment point for the tauntaun's puny T-Rex-like upper limbs. The tail is suspended from beneath the body and doesn't actually touch the ground, with the consequence that it's relatively fragile and liable to fall off if not handled carefully.
So far so good, but the magic really happens when the contents of Bag 3 come into play. First a brick-built reddish brown saddle with a couple of saddle bags is constructed (below) and it's here that sourcing the parts you'd need to exactly replicate the build starts to get a bit more challenging. The saddle incorporates a pair of reddish brown modified 1 x 2 plates with handle on side and free ends which have only ever appeared in eight retail sets. Worse still is the use of two reddish brown modified 1 x 1 plates with horizontal clip. These are used to attach the saddle bags to the saddle, and according to Bricklink they've never actually appeared in a retail set, so you might need to substitute these with equivalent elements in a different colour unless you have access to a LEGOLAND model shop.... I'm afraid that it's a similar story with the pair of reddish brown modified 1 x 1 plates with vertical clip used to attach Han's pack to the back of the saddle - these have also apparently never appeared in a retail set, although thankfully a close variant is available in reddish brown.
With the saddle complete it's time to build the tauntaun's head and carefully mount it on top of the neck. It's quite a job to wedge the pair of light bley 1 x 1 tiles into the clips on the top of the head to form the ears, and it places considerable stress on the clips which is why LEGO defines this as an 'illegal' building technique and doesn't normally permit its use in official sets. With the tauntaun now complete, all that's left to do is build the intrepid rider who is of course Han Solo. Han's dark tan legs are attached to his torso by way of a pair of 2 x 2 turntables; these allow the legs to be rotated outwards so as to adopt a more natural riding position. Once completed, Han can be carefully lowered into the saddle and handed the tauntaun's reins, at which point we're done!
For all their rarity value and collectability, I've often felt that many of the LEGO Star Wars promo items aren't especially interesting in their own right, but that's certainly not the case for this model which is superb. I've always had huge admiration for builders who can transcend the crudeness which is inevitable at this scale and craft models which are elegant and immediately recognisable, and I'd say that this is a perfect example of that art. On the downside the build got a bit fiddly towards the end, and the completed model isn't as robust as a typical retail offering, but really these are trifling details; it's a lovely display piece and it'll take pride of place alongside some of my other LEGO Star Wars favourites.
I'd like to offer my thanks to fellow Bricksetter Mark a.k.a. smokebelch for selling the set to me for an amount which, while admittedly not exactly cheap, was still a long, long way short of the ridiculous sums being asked by sellers on Bricklink; in case you're wondering, at the time of writing there were only two of these sets listed for sale on Bricklink, and at $1,995 and $3,500 respectively they might remain unsold for a while yet.... Unless you're a lottery winner, your best best would be to just source the 336 parts needed to build the model yourself - the vast majority are fairly common and inexpensive, and those that aren't can be readily substituted by alternative parts; I'll aim to scan the instructions and upload them to the Gimme LEGO Flickr stream here over the next week or so which will make it easier for you to build your own copy if you wish.