I do not know much about air conditioning units. A couple years ago my central heat/air conditioner acted like it was running but did not cool the house. I called a repair company and they said the inside unit was bad. They gave me a quote of $3,000 to fix it. They did replace the unit and said the outside unit probably won't last much longer, but being a single mom, I did not have any more money. The repairman said he would put a part on it to help it run (another $100). I agreed. The air conditioner worked great for about 1 month, then it stopped working. I had to go without the air (Texas summer) until I could come up with some money. I called the same company to come give me a quote on the outside unit. They came and showed me a pipe with a whole in it. They said because the unit was old, when the got it running previously it was shaking and something went against this metal pipe and put a whole in it. I did see the whole in the meatal pipe. They gave me a quote of another $3,000 to replace it. I said I would call them the next month when I had the money. They charged me another $100 to give that estimate. I feel like I have been taken and don't know what to do. There is a Habitat for Humanity store that sells outside units for $400 and I would love to buy that one to save money, but someone told me I had to make sure the inside unit and outside are compatible. How do I know if the units would be compatible?

but someone told me I had to make sure the inside unit and outside are compatible. How do I know if the units would be compatible?

The compatibility issue is really not your problem since you are not in the trade. When you are dealing with 400.00 used equipment costs that is a mistake waiting to happen to start with because you will have to pay $$$ to get it installed and that is not cheap to start with plus there are various tricks of the trade to make units compatible if they are not. Naturally this has to be done within reason but I have no information on your present unit to start with and you have no idea what you new used outside unit will be. There are web sites setup by the manufactures for this purpose but it will do you no good to look at them since you have no idea whet the outside unit is going to be plus doing a comparison for a non HVAC person will just be confusing to start with.
Not knowing the exact situation I do feel that you may have been taken advantage of but I have only heard your side of the story and there are always two sides to every story.

The $400 unit available to buy is a brand new RUDD.The reason I was asking about compatibility was because the sticker on the new inside unit said the metering device must match the outside unit. My inside and outside unit used to both be Trane but the new unit that was installed is a Payne. What do you think is a fair price to have a new unit purchased and installed?

For a new unit for 400.00 including install is a steal. Get it now before they change there minds. The professionals that do the install will know what to do to your inside unit to make it work correctly.