Buying a townhome for 140k

Putting 10k (more than likely less)

ARV will be right at 180k

With capital gains is this margin too small to do a flip on this property?

How much are you putting in rehab costs? If you are putting less than 10k, then I would say it's worth it. But if you need to put in 20k-30k, then that's pushing it and there's not much value to add. Also don't forget to factor in some cost overruns because those can really kill the deal. If your rehab costs are more than 10k I would look for another deal with a bigger spread. Good luck!