Question about junk yard parts - TC 242

My 99 limited 4.7 247 TC is shot, only works in 4 low.



First time I will be buying one from a yard, then have my Mechanic install it.



Yard said they have a Year 2003 242 with rear drive shaft for $300.



Question is: They say it has 99K on the 242 TC, how would I know?

Can't they just claim any mileage they want too..?



Yard does have a 6 month warranty it that matters.



Also, if anyone have dealt with used parts dealers, are they flexible on price?



Thanks