Car Guy:
I have those exact shelves - Probably over 100 feet. If you install correctly, they will hold anything. I will detail more below. Bottom line: they are a good deal and order directly from folks that make them (you are ordering from them on e-bay, but there are fees and thus you probably will find direct order more efficient).
I also have Glenn Snyder shelves (12 feet) and they are really nice. Here's the thing - Glenn Snyder are quite expensive to ship. The reason is not becasue of the seller 'marking it up', it is becasue they come in 6 foot lengths. USPS won't send and UPS considers that oversized, so they charge a premium. I believe if you buy quite a few shelves, then the cost is reasonable, but if only buying 1 or 2 shelves, that could be a problem. Glenn Snyder shelves have NO FLEX, are extremely well made and look beautiful.
The ones you linked above are made by a person in NW PA. Excellent service. I have made at least 5 orders. Every time I tell them via a note in the order form (all done online) exactly what lengths to cut, where to drill holes, and it shows up precisely as ordered. These shelves are 4ft max, so they can go USPS and the flat mailing charge is $6. I would go directly to their web site: http://www.mrtrain.com/
As far as flex goes, yes they flex, but that only happens if you pull on them. But I actually mounted mine to drywall using 50lb wall anchors. Initially I used 3 per 4ft length, but upped to it 4-5 as I added length to each row. The reason is not becasue I feared faulure, it was because the more anchors the less flex (trial and error on my part). The anchor is on the 90 degree, verticle back lip. You will notice that via a cross section it only goes up, thus the shelf can flex slightly if you try to push with your hand. But the more you anchors per shelf, the less give the lip has thus the more firm it feels. Again, found out via trial and error. I am not concerned about trains falling off with the shelves I anchored with 3 bolts as they will hold quite a bit of weight.
The Glenn Snyder shelves are more like an i-beam, so they just do not flex. Not a big deal for me (the flex) as I noted how I 'fixed' it with the MRTrain shelves. If you have studs, then drill into the studs and perhaps drill extra holes between each anchor (you'll do 4 stud anchors per shelf if your walls are studed at the standard 16" widths); you can then use the 50lb wall anchors as supplements between the 4 'standard' width anchors into the studs.
If you want me to post pictures, just let me know.
And I am so far fine - nothing has fallen off. The shelves will not break and there is no way the heaviest of engines (multiple) on the same shelf will make them break as long as you have the proper (recomended) number of anchoring points. In my case the only reason they will pop out of the wall is if the the wall board failed. I guess it could happen to me, but it would be installer error, not the shleves' fault.
And send me your address - I can mail you a 2" wide sample that I have (was left over from my last order).
Hope that helps.