You can't possibly predict whether an image will be a best seller or not. If someone wants to pay for exclusive rights, then I'm going to say it must be worth something. The price, well, you will have to decide.

Some people here, including myself, have had "sell the rights" requests. Prices have been set at $1000 and it was too much. But I personally was not prepared to give up rights for less. Maybe you are.

As far as removing similars, can you write up a custom agreement to dictate the terms or is someone else telling you the terms? If you are writing the terms, I'm going to say that image only is exclusive. You uploaded similars before this request, so if all similars need to come down too, you will have to add that cost into the equation, too.

And yes, I believe images can be removed from most sites within 72 hours.

Just my two cents. Probably worth less. 

I has not been selling so I am lucky that this buyer found me maybe $250 is better than nothing.

You could be right. If anyone wanted to buy exclusive rights to any of my photos, all except say my top 20 sellers, I might consider that price. It's kind of a personal decision, you have to weigh all the factors and make a choice. For all you know, $250 could be too much for the customer. They might be thinking more along the lines of $50. Good luck in your negotiations, it's always nice to get these types of big sales to compensate for all the other pennies we get.

I think it's the lowest amount I can accept for exclusive license. I do not understand term "full ownership" here. If buyer cannot claim copyright of the photo and cannot resell it. It seem like extra-extended print usage license.

I think it's the lowest amount I can accept for exclusive license. I do not understand term "full ownership" here. If buyer cannot claim copyright of the photo and cannot resell it. It seem like extra-extended print usage license.

Plus You can't sell it anymore. Not necessarily a horrible condition when seen as rather similar to that of Salaried photographer for company, organization.