Here's what I would recommend:
Have one auction at the beginning of con, when folks still have money. Have this include things that would be accumulated all year prior to con, and maybe even some schwag left over from the prior con. Have things that would be of value to the purchaser DURING con, such as save one ticket for the VIP dinner for this auction, maybe gift certificates for 2 for some nearby romantic restaurant, maybe the first item(s) screened with the next year's mascot, posters with this year's mascot signed by all the execs--things that would make good show & tell, maybe some cosplay props....maybe special privileges like being the first ones admitted into the dance or cosplay (if that could be worked out with yojimbo), maybe a gift certificate for room service for the con hotel........Then have another auction at the end of con of stuff exhibitors and guests present at con have donated, autographed, or whatnot.
In fact I'd have a 3rd auction as a silent auction held either at a general meeting, or it could be a month-long process online with a dedicated web link with photos of each item.
Personally I'm all for grouping little prizes into packages with themes.
A peace group I was with for years would group prizes regionally into themed vacation packages. This wasn't done as an auction (though imho it should've been) but was done as a raffle, and it always brought in a lot of money and made winners very happy.
Raffles are another way we could potentially make $ off extra schwag (both made by us and donated to us). These could be held at con, at meetings, potentially by snail mail or online. And it would be fun making themed packages out of them.