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 pour le concert de XYZ. Si vous êtes intéressés, envoyez-moi un message au 06xxxxxxxx.

instead of:

Je dispose d’une place 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 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.

I’d go for ‘tutoyer’ as your ad is clearly aimed at an other student. I never heard any students ‘vouvoyer’ when addressing each other. Relations between students are less formal than those between other people.

In ads for selling products I’d rarely use ‘tu’ or ‘toi’.

As someone from Quebec, I’ve been told we are more casual about using tutoiement than our cousins across the Atlantic so, keep in mind, my advice may be more applicable for North America.

The other websites focusing on tone and formality are right.

If you are a college-aged student looking for a flatmate, then using vous might make it sound overly formal. You may want to stick to a more familiar range. Using vous could come across as being unpleasant, too strict, etc. Tutoiement is a good way of broadcasting informality and closeness. For a product which wants to appear cool or if you want to be more emotionally engaging, it might be the right choice.

At the other end of the range, you might want to convey professionalism or formality. Then, vous is the obvious choice. It’s also the safest bet if you rather err on the side of caution. In conversation, we typically use vous by default unless given permission. We err towards politeness. As the plural form, it’s also easily perceived as addressing to the crowd rather than to the person watching – hence my previous comment about closeness.

In your specific case, There’s two clears paths:

1. Going for an informal engaging ad showcasing personality:

“Tu te cherches un appartement, situé pas trop loin de l’uni, avec des colocs trop sympas ? Tu peux arrêter de chercher maintenant. Nous sommes les droid… les colocs que tu cherchais ! La traversée du désert est terminée. T’es arrivé à bon port. On cherche un troisième pour notre 5 et demi. Grand, spacieux. Chauffé ET éclairé. Nous sommes assez calme et un peu geeky, mais on a d’la jasette.”

2. Going for a more formal ad, showcasing a serious attitude:

“Nous sommes présentement à la recherche d’un colocataire pour notre grand 5 et demi, situé à quelques pas de l’université et d’une épicerie. Chauffé et éclairé, il est à vous pour seulement 400$ par mois. Si ça vous intéresse, nous sommes disponible en soirée pour des visites. Merci.”

Both ads are likely to attract a different clientele so choose wisely.

For a “French-of-France” (i.e. fr_fr) advert, I would stick to tutoiement for two reasons:

1) People looking for sharing a flat are usually young, so they would prefer tutoiement

2) In such an ad, you actually talk independently to each candidate(s). Usually, you will look for one or more single roommates, hence the tutoiement. If you were to search for couples, you would use vouvoiement instead because they are more than one.

‘Vous’. Not because you’re addressing more than one person, but because you must not tutoie people you don’t know. Question de politesse. Quelques publicitaires essayent de ‘cibler les jeunes’ en les tutoyant, mais c’est une ruse grossière.