ok i bought my car from a junkyard as a reposed car then 4 months later the transmission started acting wierd so i got a quote for a rebuild of 1400 total price, but the junkyard where i bought the car said they would give me a deal on a transmission
it has 100000 miles on it and he said i can grab it for 275 and i found someone who will put it in for 350

what should i do

275+350=625= $$ of 2 junkyard transmissions. Probability of one being maintained properly seem good. Dig into the history of the donor-car for maintenance records/previous owner. If possible, call previous owner. Maybe it's: "better the devil you know", spending the 1400 hard-earned dollars. Check the yards' reputation. Most yards assume what they are selling is near-perfect and will work for a long time.......
If you opt for the rebuild. Check the rebuilders reputation also. Ask if they use OEM parts or generic parts. Check their Warranty...actually read it/ask questions about what your responsibility is regarding maintenance/etc. Find/ask previous customers of that rebuilder.

The good thing about the rebuild is, they usually come with a warranty.

If that junkyard tranny is bad, which you won't know until you get it installed, you'll have to buy another tranny and pay for another installation. Then you'll be almost up to the price of the rebuild.