I'm sorry you had such a bad experience trying to sell to buyers using a USDA loan. My husband and I have purchased two homes and looking to purchase our next home using a USDA loan. We have also advertised each time that both houses are eligible for USDA loans and we have no problem selling to buyers using a USDA loan.
We have loved purchasing our homes with USDA loans; in fact we see it as the best kept secret! I tell all my friends looking to buy a home about this option. We have the income to make the monthly payments without any problems but are not able to come up with a large down payment. USDA loans do take longer to close. I believe both of ours took 6-8 weeks compared to what you usually hear of 30 days or less. It can be frustrating to wait but shouldn't be any problems with the buyers closing if they do not do anything to wreck their credit after the pre-approval.
Both of these houses we purchase are in Ohio so there may be differences in other states just because the loan is being processed through a state or regional office. With our current home, the Columbus office had their computers crash and they lost all of the work they had done on loans but they worked quickly and it "only" backed us up about 2 weeks. Any realtor working in areas that are USDA eligible SHOULD be knowledgeable of USDA loans and, I feel, has an obligation to warn both the seller and the buyer that it will take longer to close. With all the information, the buyer has a right to accept or reject the offer.
I will also note that one of the homes we purchased was through a dual agent. I can see how this can be an issue at times but fortunately our process went smoothly and it was one of the homes we purchased with a USDA loan.
Doing your homework is always wise but I wanted to let everyone know that USDA loans can be great and your bad experience is not typical.