If you want a named cultivar you will need to buy plants. They won’t come true from seed. Only the species will do that.

The place to go for hard-to-find seeds is Ebay, where almost everything is sold.

Unfortunately, many sites on eBay are scams. Some even offer plants which don’t actually exist. Unless you really know what you’re after and can check out the supplier it’s east to get burnt. We get many posts on GW from people who’ve been sold weeds or common plants when they thought they were getting something rare. To be honest, eBay is the last place I’d got to for seeds unless I was desperate and/or had a recommendation. A specialist seed company is a much better bet.

And for this particular plant seed is no good unless you are happy with the straight species. Anyone offering seed of cultivars is a fraud.

I was about to make the same comment but Floral beat me to it. Unless you are dealing with a vendor you know to be reputable, buying seeds on eBay is a very risky business!!

What is for sure is that the prior two comments are made by people who DON'T buy seeds on Ebay! I have bought so many over the years and literally have had no problem at all. Whatever weird plant I want, it's there. Now, I have bought plants that were OVERPRICED there, but that's the worst of it. St. John's wort is not going to be anything rare. One could probably find some certified seed distributor that will sell the seed, and by the time shipping is paid it will cost $20. Or one can pay $4 on Ebay and take a chance. And throw it away if it turns out to be some ghastly horror story. (And still get their money back!) But I doubt that would happen. Also, while there are mom-and-pop seed vendors on Ebay, there are also professional, full-time seed distributors who are simply expanding their exposure. It's a very active seed selling market.

"It's a very active seed selling market.".........where you can find seeds for named forms of Japanese maples and other plants that are just NOT propagated by seed or come true to type if seed grown. Being potentially "overpriced" is NOT the worst of it!! Outright fraud is being committed to those who are not horticulturally savvy enough to know what can and can't be grown in this manner.

Consider the source......and caveat emptor.

Where in life does one get to throw away their brain and prosper with no understanding of the basics? One can't even trust GPS complete, as surely, at some point, it will take them to a wrong address! Yes, as a buyer of anything, a person has to make rational judgments about what they are being told. And it is no different with an online marketplace. A person must read the listing completely and have an understanding of basics. Since NO seeds, anywhere in the universe produce clones, we can grasp that seeds of clones sold on Ebay will produce "children" of clones. It will be the species and likely will, at least, have a fair amount of characteristics of the parents. The main thing is not to remain a clueless idiot in life and then one will be fine.

‘St. John's wort is not going to be anything rare.’

Except that this is, at least as seed. It’s a particular Hypericum species which does not seem to be readily available. I could only find one source, the same as the OP, and it isn’t listed on eBay U.K. at all. Whether, being a N American native it’s available there I don’t know. Perhaps some suppliers of native plants could help.

Fwiw I have purchased from eBay but only when there’s been no alternative and from dealers who can be vetted. As I said before “Unless you really know what you’re after and can check out the supplier it’s easy to get burnt.” It’s not simply a question of the money. With seeds it takes months of work and waiting before you realise you’ve been duped.

One may be assuming too much :-) We see countless threads and postings here and on other forums from folks who have obviously not done much serious gardening at all, have no idea what they are doing or what they are asking for and give little attention to even basic planting requirements, let alone botanical distinctions between species and cultivars and the difference in necessary propagation methods. And these are the types of customers that get suckered in by sites like eBay.

"The main thing is not to remain a clueless idiot in life and then one will be fine."

Oh, were that to be true!! The fact that many are and will remain so is why we find ourselves in our current social, political and health predicaments!!

This species of St John's wort also appears to be endangered in parts of its native range so seed availability may be limited as a result.

Jojker, have you sought out local native plant nurseries to see what they have? Smaller seedling starts might be a better approach, especially if you want a presence in a single season. You are not going to get that from seed!!

it still possible even though it unlikely

And for landscaping, it's probably not the best idea for one to fixate on something that is rare. Better just to solve the problem with something that is available.

saint John'wort is an anti depressant