Screenshot showing the Human base (Pencil Sector, White dot) and Guardian Systems relatively nearby.
https://imgur.com/a/LierMm0
Rational:
1) Fdev like reusing mechanics.
2) The BGS is transactional, and is already logging the selling of Relics etc. (Just like UA/UP etc)
3) Both Thargoid stuff and Relics have similar Nano-bot tech that can mess with our Human Tech.
4) Relics do not seem to have much other purpose yet except at Thargoid Structures.
5) The selling in the Bubble may be too far away from other Guardian build nodes in their FTL comms network, or there maybe too much human tech to mask any signal.
6) Humanity is an ant compared to the Thargoids. The Thargoids have been apparently holding back their full capability (fear of Mycoid 2.0, or maybe a matter of efficient use of resources) . (They are periodically ratcheting up their firepower. This is possibly in response to human resistance, but that could be up for debate.) When you are a ant in a tank battle bringing in another tank that has a grudge against the first tank is not necessarily a bad idea. (Thargoids seem to have the capability to curb stomp humanity if they choose to, but have been surprisingly constrained given their supposed capabilities. )
Addressing Weakness in the Idea:
1) The affect on the game of Guardian involvement would not necessarily require a ton of work by Fdev. The response could manifest by thargoids spawning only closer to the Pleiades, maybe a Galnet article commenting on the change, and speculation on what has pulled the thargoid's attention away from the bubble. Guardian ships etc would not be needed right away. Even if they only help us develop anti-thargoid weapons without being directly involved in the conflict that would be a huge win for humanity.
2) We are already using Guardian tech in our anti-xeno weapons based on what we were able to gather from half broken scraps. Without the Guardian weapons it would have been near impossible to have even the minor success against the thargoids that we currently enjoy. Imagine being able to talk to the people that actually designed it. However, using Guardian based tech in our ships would make us very vulnerable to the Constructs/Exiles. ( Yes it is high risk high reward, but what does humanity really have to lose when faced with the thargoid threat that can cripple starports, and Capitol class ships with ease?
3) The constructs were not united in their destruction of the Guardians. They have had millions of years to think about and regret their actions. (or patch the bug in their code if you will) They may be looking for a way to redeem themselves. (The Military AI would be the most helpful to humanity, and they did not want to destroy the Guardians) What ever the thing that Halsey ran into seemed nice enough, and could fit the description of a advanced AI species.
4) Mycoid slowed the Thargoids down last time, but it is unlikely the Thargoids will not have adapted this time. (They found the INRA mycoid bases, and apparently some of the aegis bases as well.)
5) The Thargoids are abducting people/ escape pods. This should be very concerning. If they are preparing their own brand of Mycoid/Plague to unleash on humanity the Guardian's bio skills could save our skins.
6) Without outside help or some doomsday weapon being built in the background by the human powers that be to use against the thargoids we will be at the thargoid's mercy to survive at all. Although the war like nature of guardians is concerning (as is the Constructs past actions) We are already using their help indirectly via studying their tech/ruins, but without more help we cannot hope to outmatch the thargoids in a conventional conflict.
TL DR:
It may not work. Even if it does it may be bad for humanity, but we have little to lose at the moment. If people are out at the Guardian Ruins, and can dump their unneeded Guardian stuff at the base before returning to the bubble that would be great. If we cannot find allies, or more advanced tech we are likely going to be curbed stomped anyway. (we might even gain the ability to talk/negotiate with the thargoids. The lore is not consistent on if humanity can communicate with the thargoids as there is mention of joint bases/missions etc in the old games, but if such tech existed it would be used to communicate with the Thargoids. )
Edit: If you don't want to leave the bubble it is possible to "acquire donations" from NPCs of Guardian items. They are basically everywhere, but the Dark Wheel planetary bases in Founders World seem to be very good locations.