If you can't sell the cars to Los Santos Customs because their individial worth exceeds 50k then you are out of luck. You will not be able to sell them anywhere. You will only be able to get rid of those cars when your garage has hit its limit and you will be asked to replace a car. This question and its answer are related to this problem.
On the topic of not loosing cars upon buying a new property, if you buy a property with the same amount of garage space it will not result in the loss of a car.
If you buy yourself a property with less garage space than the one you had before you will loose every car that is in the last few slots. As an example: When currently owning a 10 garage space property the first 6 cars in your garage are important and the last four are the ones you will loose upon buying a 6 garage space property.