You've hit on a source of great agent debate and frustration in our area! Some of us contend that it should not matter whether or not food or raffles are provided. Agents need to learn the inventory and broker open houses are a nice opportunity to run around town for two hours, and see as many properties as we can in a relatively short period of time!
Yes, we can "preview" too. This means having to call to make an appointment and asking the seller to allow a special showing. Broker open houses are really convenient for both parties!
I personally have spent well over $100 on food only to have a handful of agents attend. (And yes, we do promote them via email, fax, MLS, etc...) So I don't necessarily buy into the idea that "if you feed them, they will come".
If the house is truly different than most on the market, we can count on seeing more agents attend. If it is similar to most in your area, or located in a condo complex where many units are similar, agents are not likely to show up due to the limited amount of time and the number of homes available to view that day.
I will always still offer a brokers open...and keep my fingers crossed. It's just one of many elements in our marketing plan.
I have always paid for the food myself and feel it is part of the marketing plan. But whether or not the seller is "obligated" to provide food depends on your agreement. But as I said, it may be a moot point depending on how well broker open houses are attended in your area.
Best of luck!