The simple general answer is that:
If you hold its forbidden but they hold its allowed, then you may help them to do it.
But, if you hold its allowed and they hold its forbidden, then you may not help them do it.
It all depends only on what they hold, not you.
Explanation and sources with more specifics:
The Torah in Vayikra 19:14 says "...you shall not place a stumbling block in front of the blind.." Rashi brings the Sifra which explains it to mean that one may not give bad advice to someone. The Rambam (based on Gemara) explains it to mean one may not help someone sin.
The K'sav Sofer in Yoreh Deah 77 says that if the other person holds something is permitted, then you may help them, but if the other person holds it is forbidden, then you may not help them. It is obvious that helping them do what they think is forbidden is bad advice for them, causing them to sin even if you don't think it is a sin for yourself. However, if they don't have an issur, you are not harming them at all with any bad advice. (even if you wouldn't do it yourself)
Reb Moshe Feinstein (O.C., I.186 by esrogim of shemittah, and Even HaEzer 4:61, by heter mechirah during shemittah in Israel) says that if the other people rely on a legitimate poseik for a heter, then you may supply them with whatever they view as heter, even if you yourself hold it is forbidden. Rav Shlomo Zalman Auerbach also agrees with this. (Minchas Shlomo A:42,44)
So clearly you may help them hooble and sell them a hoobler device. The fact that you yourself do not hooble is not important. This is because their reliance on the hooble permitting poseik makes what they are doing absolutely not a sin. Tempting them to do it is also not bad advice. Therefore, it cannot be lifnei iver. :)