How to decide whether to Accept an offer or Counter?

[Janelle Hicks]

So i recently realized a house I purchased came with an extra, already subdivided lot, that I can sell separately. It is a small lot, but lots sell well in my area. The cheapest other one in town is 20k, but it is larger than mine. We listed it for 17k, my realtor said to expect to get anywhere between 10-14k, minus closing fees of $3500. It was listed Tuesday, today (Friday) we got an all-cash offer, quick close for 10k. Our realtor said she thinks it's a good offer and does not think the buyer will want to go higher due to the small size of the lot.

I guess my question is with these kinds of things, is it best to accept the first offer? Better to counter? Better to wait and see if something better comes? I haven't ever sold property before, other than our home. I sort of want to counter at 12k just because I don't want to just accept the first offer. But seeing as this land is just sort of an unexpected bonus to an already good deal, I don't really have a target for what I want to make on the deal, and quick and easy sounds great right now when I have lots of outstanding debts from the rehab of the house.

Is there a good mathematical way to decide? Rule of thumb?