This is an important warning for people purchasing seeds from an unsure source. That is to say sellers that produce the seed themselves as opposed to those selling on well known brands.
I'm sure that there are plenty of honest and/or well informed people selling perfectly good seed on ebay and it is not my intention to poo poo them in anyway, but there are dangers that you need to be aware of before risking your hard earned pennys.
Certain plants are inherently promiscuous and will happily cross pollinate with members of a different species. My particular experience came with the Cucurbit family (Squashes/Pumpkins/Cucumbers/Melons/Marrows/Courgettes/Zuchinni etc). If any of these are raised together any seeds produced will most likely not be pure (bees don't pollinate by species). Special precautions have to be made. For instance isolating individual species or cultivars and pollinating by hand or not growing a crossable type within 500 meters of another.
There is nothing to stop Joe Bloggs buying a Squash from a shop, cleaning up the left over seeds(a service in itself, its messy believe me) and presenting them for sale. The problem is you have no way of knowing where it has come from and what its been grown with.
Last year I purchased some squash seeds from a seed seller, I had four types. Two came true more or less, one had signifiant annomillies and one was clearly crossed with a pumpkin and of no recognisable cultivar and although edible it was not fantastic and most went on the compost heap.
In conclusion: ask questions when buying any seeds. Eg What processes did the grower go through to obtain the seed. If they are vague or evasive, avoid. There are plenty of reputable seed producers in this country, just use your favorite search engine, and those suppliers will have strict quality control due to a competetive market. And if you are buying second hand seed make sure its in date, some lasts longer than others.
Hope this helps!