When selling things sometimes (or even buying, but rarely) It will show the error message saying basically 'you don't have enough money'. However, even if I have a load of money and am only spending like, a cent, it will still show until I refresh a few times.
Mainly, however, is selling land. It refuses to sell, at any amount of land. I have about 10 miles now (Yes I know, teh noobeh) But still, I try to sell even 0.005 land, nope.
Also worth mentioning is the error message itself, which could use a bit of rewriting: "Either you do not have enough land or Dollars to perform that transaction or you attempted to purchase more than the maximum amount of land available to purchase at one time is 20 miles."
Somebody more involved please take note of this and see if its the same on your comp (My firefox has been a bit bugged since I did some stuff to speed it up).