Cartels is the main danger with unconditional basic income. If you make sure everyone can eat, you create a moral hazard, where the insured are not sensitive to price anymore. Having said that, we already have means-tested welfare and food stamps, and cartels of fruit stores don't spontaneously appear everywhere. However, they may appear in food deserts and other places where there aren't a lot of participants. Competition is the only factor I can think of to combat this. But in some areas with few participants, this collusion can ultimately happen.
And then of course there is this result: https://www.arxiv.org/abs/1201.3798v2
Does anyone here have constructive thoughts on how to overcome this problem? Price controls and collective bargaining via single payer is all I can think of besides competition.