Who pays for wedding reception guests? Is it common for them to pay for themselves?

The buffet fee of my reception is $55. What is the tradition on payment? Wedding guests, such as friends and coworkers, of my fiance said they'd be willing to pay. But I recall attending receptions where I was free of charge. Is it common to have them pay? Is an rsvp payment normal protocal?
Who pays for wedding reception guests? Is it common for them to pay for themselves?
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