This is a problem for me, too. I'm interested in a lot of different stuff for my layout, but without seeing it in person I'm reluctant to put $500 into a locomotive. For instance, I am considering buying Piko's latest BR 24... but until I actually saw it at a train show, I wasn't going to buy it sight unseen. The same is true for rolling stock--some of it is very nice, some toy-like--and how can you tell without seeing it in person? And from a seller's point of view, I can say that every time I've visited the one G-scale-carrying shop nearest me, I've walked out with at least one thing I didn't know I needed
Here's my take on this one: I've seen people offer "kits" that cost two or three times the cost of a RTR car. For some folks, that's not a issue--because the item is only available as a kit, because money is no object, or whatever. But I'm not interested in paying $300 for a rolling stock kit that I have to build, paint, and letter. Maybe it's just me, but I just can't see it. Especially since I like to run two or three of each car type.