IF you have advertised semen from your bull(s) and someone inquired about it then it might be legit. If not, then probably a scam, especially if it is not a verifiable address or contact. If you are owner of a prominent major bull and people know about your program/ranch, then it is possible that they might be making a "cold contact" to see if you have semen to sell.
On a sidebar...if ANY "offer" includes offering you more than the asking price, then in all probability it is a scam. The "Nigerian Cartel"...lol...is very adept and rampant at these sort of things.
Two other warnings: If the "prospect" offers MORE than asking price...SCAM. If the "prospect" asks you to REFUND the excess and/or send excess $$ to a third party...SCAM.
IF the person inquiring has a phone number and you can't locate a name/address via "Google" search, then it might either be a cell phone number or a scam contact.