Competing Customers Stealing My Stuff

Suppose you are in a rummage sale hosted by a church (an established organization, not just an individual) and you have in your possession items you wish to buy. Then another shopper sees what you have and approaches the vendor and convinces them to sell it to them instead of you. So the seller goes over to you and tells you that you may not buy what you have claimed because they want to sell it to another person, so they force you to give it up (since you don't own it yet because they refuse to sell to you). Is it illegal for the seller to do this? It has happened to me multiple times at different places locally (Seattle, Washington area). - Is this your question? Add additional information