The ESSENTIAL ingredient in making these events successful is that collectors will turn up expecting and hoping to add to their collections in one way or another.
This 'trade' also involves social intercourse over coffee naturally, where one's general knowledge of these things can be increased, and social contacts made.
However are there enough slot collectors likely to attend such a swapmeet with 'stuff' to sell of swap, in order to garner the attention of other collectors?
I think that is the first and most important question to answer.
If 'yes' then these events would work I think?