It depends on the source and also the genus of the bamboo. You should have confidence in your source - most of the EBay seeds are a scam. Typically, there are always a few species of common bamboo that happen to be in flower somewhere in the world, producing seed, that are labeled as ornamental species. Trust me, a couple times each month, someone emails me pictures, or brings in plants, of their EBay seedlings questioning why their bamboo looks nothing like the pictures on my nursery website. P.T. Barnum had the answer to their question but it seems kind of mean to call someone a sucker. :(
The American Bamboo Society www.americanbamboo.org has information on bamboo seeds and flowering data. Sometimes seeds from a positively identified source can be obtained through the ABS. The seedlings will vary in appearance and vigor. Some bamboo genus seem to grow truer from seed than others - with all, only the most vigorous seedlings should be allowed to grow out.
Growing bamboo from seed can be fun but, even with a known source of positively identified seeds, the process should be treated as an adventure. Don't expect to grow a clone on the parent plant. If you want a clone, buy or propagate a clone.
Here's a picture of my son with a "B.lako" seedling next to a true B.lako. Both plants are the same age (1 year). Someone brought in the seedling, grown from the EBay seed source based in Germany. Even in the photo, you can see obvious differences.