Whereas the function of the passive voice is to remove the agent from the discussion of a transitive activity, the function of the mediopassive voice is to discuss an event in a way which explicitly denies the involvement of an initiating agent.
Source: wiktionary
Until about 1900 the r-element attached to mediopassive endings was merely regarded as a marginal feature peculiar to Halo-Celtic; it was taken as evidence for Italo-Celtic because it was supposed to be an innovation of these two groups.
Source: wiktionary
A systematic study of mediopassive sell in the BNC (Figure 6) confirms that – at least for this verb – the constraint seems to hold: the vast majority of mediopassives with sell contain some kind of modification, namely 92.2%.
Source: wiktionary
More importantly, it is highly context dependent: (a) certain text types (such as advertising copy) seem to license bare mediopassives more than others (see Section 6.4.1), (b) contextually implied contrast may enable an inherent property reading of a bare mediopassive (see Sections 6.4.2.1 and 6.4.2.2), or the verb itself might foster bare mediopassive use (this seems to be the case with denominal verbs like zip, button, bolt, or clamp, see Goldberg and Ackerman 2001: 806).
Source: wiktionary