I understand your predicament. With some breeds, the differences show up by 5 weeks and they can be sexed fairly easily at that point. With other breeds, the lady selling them couldn't possibly sex them. Even the hatcheries who use feather sexing and vent sexing only assure a 90% accuracy rate. Some breeds are so tough that even at the 12 week point, after you've fed them for months, the differences are somewhat difficult to detect.
If she takes them back? You'd be good. I guess if I were in your situation, I too would get a few "extras" because it seems highly unlikely the sexing will be accurate. This is precisely why hatcheries sell so many sex-links. These are chicks, of course, that display their sex at hatch. Males being white and females being red, for example. Sex linking only works in some cases, and in most cases, this is producing hybrids (mutts) which doesn't satisfy everyone's desires for pure breeds either.