Is there such a thing as a regular trade in archeotech? Or what form would it take?

In my view, any archeotech trade will be very limited at best. Why? Well, first and foremost, the Adeptus Mechanicus considers these items to be Holy. It will not just trade them away for throne gelt: if the Adeptus Mechanicus grants an individual the great honour of possessing one of the great works of hallowed antiquity, it must have a good reason for doing so, such as great services rendered. Furthermore, there is the Treaty of Mars: one could read this as giving the Adeptus Mechanicus a claim on any piece of Archeotech recovered by the Imperium. Now they won't be able to go that far (even if there are undoubtedly factions in the Mechanicus willing to go that far and further) and a lot depends on who recovers what when and where. A poor sinkhole miner who chances on an archeotech cache with a magnificent powersword has obviously a worse chance of keeping his grubby hands on it than a Rogue Trader does, but even said Rogue Trader might find the Mechanicus becoming 'insistent' if his find is truly of momentous importance. So it seems to me that the Adeptus Mechanicus won't sell (as opposed to grant) Archeotech and will do its utmost to recover any Archeotech that is on an 'open market', by any means if need be.

To this we should add that the parties that do posses Archeotech will be loathe to part with it. Whatever is in the hands of the Imperial Navy, Imperial Guard, Adeptus Astartes or any other grand Imperial organization is likely to stay there. Again, for services rendered or in pursuit of a grand goal they might grant what they have to another party, but never for throne gelt. The same goes for the Imperial Great Houses: what Rogue Trader family, Navigator House or Planetary Governor will sell of these amazingly handy and highly prestigious items? I can only imagine a situation of serious penury that would force them to do so, and then, they would probably do so in silence (which not only limits the shame of having fallen so low, but also limits the chance that the Adeptus Mechanicus starts meddling).

So we are left with an archeotech trade that depends on either the Great and Powerful becoming bankrupt and selling of some of their most precious items and the odd chance that a powerful individual like a Rogue Trader finds an item of archeotech that doesn't interest him and decides to sell it off (ask yourself how often players sell of their archeotech items), all of it overshadowed by an Adeptus Mechanicus trying to get hold of it. This is not a lot, which might make it a very profitable business for the few brokers who know who might be interested and able to pay if any item comes on the market. But while it is not illegal per se according to Imperial law (although the Mechanicus might beg to differ) and not even remotely comparable to something as terrible as the Cold Trade, it feels a bit shady and in each case, highly specialised. It demands a vast web of sector spinning contacts, a fair deal of protection and great personal skill.

is my interpretation correct? Or am I going too far in making archeotech hard to buy?

For the most part, I'd say you're correct.

I'd say that trading Archeotech, whilst extremely rare would be as you said to recoup lost funds, or to swap for other items of similar value, or favours etc.
If a group of players, don't finda use for a piece of archeotech, and it is small enough, it will end up in a trophy room.

That certainly happened with one of my campaigns, we picked up items that were very good mechanically, and cool otherwise. just Not fit for our characters archetypes. Thank you Stars of Inequity... and the Stryxis.

They would have stayed there until we went, "well lets offer this instead!"

Archeotech can be traded. But it's a market for the very rich and the very well connected. Finding archeotech takes serious contacts, for reasons mentioned above.

It almost seems like Magic Items in D&D 5e; only the wealthiest individuals, and a very small market of types, will both have interest, and the funds, to make it worthwhile, while the difficulty in finding any means the transactions are rare. At that point, you aren't really doing it for the money, but for the prestige, connections with organizations, or to assist in getting something else that you want, that money, alone, cannot buy you.

If I got a new ship, say from the Breaking Yards, I assume they've stripped out all of the extra, valuable components, and I'll need to purchase them, separately, as well as, more than likely, the part I want is no longer one they "have in stock". Say I make a great deal of my living on my teleportarium. This piece is "mundane enough" archeotech it's in the main book, no extra books to buy, but, if you needed a new one, either for a second ship, or because the heavily arcane device was damaged in a battle, and is out of order, I don't imagine this piece being at all easy to find, or being easily acquired with "just money".