The gemara has three opinions when one purchases an animal from a Jew and wasn't told whether or not it had already given birth to a Bechor. Rav says that the assumption is that the first born will be a bechor because if the it had already given birth the Jew would have mentioned it to praise his the item he is selling. Shmuel says that it is a safeik since the seller may have assumed that the buyer wanted it to shecht so it wasn't relevant whether or not it had already given birth. R. Yochanan says that the buyer can assume that it is not a bechor because there is an obligation on the seller to notify the buyer if the first born would be a bechor otherwise he would be in violation of lifeni iever (tosafos).
There is a machlokes Rashi, Tosafos and Rabbeinu Gershom within the opinion of Rav as to what the advantage would be in telling the buyer that it had already given birth. Rashi says that it would have more value since the animal born from here on wouldn't have to be given to the kohein. Rabbeinu Gershom says the advantage would be that it is an animal capable of providing offspring. Tosafos (quoting rashi) says that the advantage would be that the animal has proven it is capable of surviving child birth and therefore more valuable. The Maharit Algazi asks although when the seller doesn't say anything we don't have to be concerned that it actually gave birth to a viable child, but why are we not concerned that it miscarried (ti'nuf) which would also exempt the next born from being a bechor? He explains that according to the rationale of Rashi and Rabbeinu Gershom, the logic would apply to ti'nuf as well. According to rashi since ti'nuf exempts from having to give the next born to the kohein, the seller would notify about tinuf as well. According to Rabbeinu Gershom, perhaps even ti'nuf would be an advantage in showing that it is capable of conceiving (although wasn't able to produce a viable offspring). But, according to Tosafos where the advantage is that the animal is capable of surviving childbirth, this wouldn't apply to ti'nuf. According to Tosafos the seller would have no reason to tell the buyer about the ti'nuf so we should be concerned that it was me'taneif and the next born may not be a bechor? Perhaps this is why Tosafos offers an alternate approach.
Tosafos writes that only a Jew would be believed about the advantage of having already survived a childbirth and therefore being more valuable but a goy wouldn't be believed about this. Tosafos proves from here that even if a goy is מסיח לפי תומו, talking without trying to make a halachic statement, he isn't believed since we assume he is lying to make the item seem more valuable. The Maharit Algazi asks that this implies that if the goy was truly מסיח לפי תומו and not trying to make his item seem more valuable he would be believed even for bechor. But, the gemara says in yevamos that a goy is never believed מסיח לפי תומו on an issur d'oraysa, he is only believed when he is מסיח לפי תומו on an issur d'rabonon, but this is an issur d'oraysa? From this the Maharit Algazi concludes that the Rov of most animals having already given birth after reaching the ages listed earlier in the perek, is a valid rov on a d'oraysa level, just m'drabonon not a good rov since it is dependent on an action (gemara 20a). Since m'doraysa we have a rov that the next born is not a bechor, and it is only d'rabonon that we can't rely on the rov, we can believe a goy if he is truly מסיח לפי תומו without any motive to glorify his item.