To me, an expo with just machines to play is simply a meet or get together.

As others have said, in order to appeal to the general public there will need to be machines/parts/mods for sale.  Also would be great to have some speakers on repairs/mods/history/how to find bargains etc.

If i was to attend (which i would be reasonably keen to), i would love to stock up on some parts, have a look at some stalls where guys like Homepin have some of their replacement boards setup, have a look at some LED mods people are selling, and hopefully walk away with a few bits and pieces.

I reckon a competition is also essential to get more people interested - i have been to meets before with comps and it is a great way to get together for a bit of friendly competitiveness.

I also think machines need to be for sale - and so what if some people have elevated prices.  At the end of the day, no-one is obliged to buy anything - charge the sellers a small percentage of the advertised for sale price if it sells.  If there are a few sellers, it won't take a genius to work out who has good deals and who doesn't.