Both are acceptable but I would suggest sticking to vous because you are addressing the message to a group of people. Regardless of whether you use tu or vous at the singular, the plural is always vous.
There is absolutely nothing wrong with writing:
Je dispose d'une place de pour le concert de XYZ. Si vous êtes intéressés, envoyez-moi un message au 06xxxxxxxx.
instead of:
Je dispose d'une place de pour le concert de XYZ. Si tu es intéressé(e), envoie-moi un message au 06xxxxxxxx.
The first sentence is addressed to a group of people so the question about tu/vous is irrelevant, there is no vosotros/ustedes distinction in French.
The second sentence is addressed to a single person. Either really a single person, e.g. Pierre, je dispose d'une place pour le concert..., but this doesn't match your situation or you are addressing the message to several people and want each recipient to feel like you are writing to him/her personally.
This is possible but I believe less frequent in this kind of advertisement.
Should you have asked to a single unknown person you vouvoie, the sentence would have been:
Je dispose d'une place de pour le concert de XYZ. Si vous êtes intéressé(e), envoyez-moi un message au 06xxxxxxxx.
Finally, you can be sure nobody will be outraged if you use a singular vous which is a mark of respect while there is a small risk for people not to appreciate being tutoyé by someone they do not know personally. The latter is very unlikely to happen between students though.
