Rashi explains that when someone has ma'aser sheini money and intends to purchase an animal (which would become ma'aser sheini) to eat outside of yerushalayim, it would be a violation of lifnei iver for the seller to sell it to him. Tosafos (d.h. aval) asks that this should not be a violation of lifnei iver because lifnei iver only applies by "two sides of a river". In other words, lifnei iver would only apply if the buyer could not find someone else to sell him the animal, but since he can find someone else to sell him the animal, it is not lifnei iver for this seller to sell it to him.
Although Tosafos in Shabbos 3a understands that there would still be an issur d'rabonon of supporting and aiding in the doing of an aveira, even if the seller would not be in violation of a real lifnei iver, he should still be in violation of an issur d'rabonon. Perhaps Tosafos is assuming that the buyer we are speaking has a status of a mumar, and based on Tosafos in Avoda Zara [explained by the shach (y.d. 151:6) ], the issur of supporting in doing an aveira wouldn't apply. Furthermore, based on dagul m'rvava there, the issur of helping do an aveira wouldn't apply to anyone who is intentionally doing an aveira, which is certainly the case here. Also, it is possible that Tosafos held that for an issur d'rabonon we would not impose such a penalty on the seller.
It seems to me that Tosafos question on Rashi can be answered based on the Mishna l'melech (malveh v'loveh 4:2) cited by pischei teshuva (y.d. 160:1) who explains that one who borrows with interest violates lifnei iver by causing the lender to do an aveira, even if their are other borrowers who are willing to borrow from this lender with interest. His rationale is that the fact that if reuven and shimon are the two potential borrowers, they can't each be moreh heter saying that it is not "two sides of a river" since the other can borrow b'issur. Just as when reuven sees shimon about to pass wine to a nazir and quickly jumps to pass it himself, is still in violation of lifnei iver, so too when reuven sees shimon about to borrow with interest and jumps to borrow himself, would be in violation. Here too, the fact that the buyer can purchase from another seller who would sell it to him b'issur would not help to remove the lifnei iver from this seller.