Evaluating Unique (truly one of a kind) Properties

[Julie K Fredrick]

I am struggling on understanding the value of a historic 1362 sf cottage in an historic town in the South. The cottage is in pretty rough shape. My realtor pulled comps, but I don't feel they are an apples to apples comparables. The sellers purchased the property 2 years ago for $210K. They have done nothing to it. The market in the town has been somewhat flat, but rising slightly...maybe 3% per year. My realtor said it's worth $220K-$230K max. Asking price is $270K, reduced from the original asking price of $283K. It has been on the market for 65 days. 

Today I offered $225K. The verbal counter is $260K. I am nervous going any higher than $235K because it needs around $50K in work to bring it to where I want it to be for an airBnB. To my shock and surprise my realtor now feels the property is worth $260 (only 4 days ago she pulled comps and told me it was worth the $220K-$230K max!). 

I want the house as I think it will be a profitable airBnB due to the location in the historic district, but I'm not in love with it, so I can stay objective. But I feel very lost. It feels like I'm buying a one of a kind antique. 

How do I properly value this cottage. Is it a gem or a money pit I'll never get my money back out of?

Thanks in advance!

Julie