> Buy goods/services from your friends

Certainly you are _allowed_ -- but if it's for an economically meaningful service, like renting an apartment, the follow-on suggestion that it's "only weird if you make it weird" really fails to capture how likely it is that it _will_ change the nature of your relationship as soon as someone decides not to pay, or whatever other irreconcilable issue comes up. Just be careful when going into business in any way with people you otherwise like.