I don't usually post here, but this is a good question at this time of year when seeds are maturing and we are all out in the garden collecting. Not all seed sellers on Ebay know that the seeds they collect from hybrids will not come true, or maybe they don't care in some cases.
Hybrids do not come true from saved seeds. Some hybrid populations eventually stabilize if you keep selecting only plants with the characteristics you want generation after generation rather than performing the original crosses each year, but seed companies typically do not do that and it can take years of selection to acheive.
If you save seeds from hybrids, the plants will exhibit a range of characteristics based on the backgrounds of the parents. You will likely get some plants that are exactly like the plant you started with, but you will get a lot of variation as well. I sometimes use seeds saved from hybrids and the first generation is often pretty good if you don't mind some surprises and are prepared for it. If you need uniformity of size, color, height or color, don't bother. You may also lose other important characteristics like disease or insect resistence, improved bloom time, etc. If you continue to save seeds from those plants, the resemblance to your original will deteriorate each generation until there is little or no resemblance.
I have purchased seeds on ebay for several
years. Unfortunately, I purchased seeds this year that were not at all true to type. Yesterday I looked at a pricey hybrid variety I wanted and rather than bid, I emailed the seller and asked her how she got the seeds. She answered that they were fresh-she just saved them from hers. I won't be bidding on those.