Is it a Banksy piece, though?

No major auction house will touch pieces ripped from the street, in part because these will usually not get a certificate of authenticity from Pest Control. IF a certificate of authenticity were to be given for a removed street piece, it would confirm not only that the street work was originally by Banksy, but also that the removed object is still to be considered a work of art in its current form. From a market point of view this is important because site-specific artworks, which the donkey painting is a clear example of, are arguably ruined once removed, and so should no longer necessarily be considered a work of art by the initial creator.

Most collectors would be apprehensive about paying large sums of money for work that is not authenticated. Add to the above question about the object's questionable status as art the concern that the wall piece for sale could be an outright forgery.

In lieu of formal authentication, sellers will often provide other types of provenance, e.g. images of the removed piece at its original site, images/video of the removal process, documentation on the artist's website/social media of the work being created, etc. While such documentation may seem convincing, it does not remove concerns about the object's status as art created by a specific author (rather than simply a piece of wall with some paint on it). It also does not ensure that the object the buyer receives is not a forged copy of the original street work.

These are some of the reasons decontextualised street artworks tend to not fare well on the art market. Another reason, as stated by god’s ultimate noise, is that most people who are into this type of art believe that street artworks belong in the street where they were made.

Edit: I wrote a book chapter about this a few years back, which is freely available here.