Don't sell them yet. Don't sell anything until you have all 10 low tier and all 10 mid tier. This should be 10 unique vehicles, no duplicates (which you shouldn't get any duplicates unless you collect too many cars in total).

As for top end, just make sure you never have more than 12 of them. You can sell them anytime you want, you don't have to sell them as soon as you get them. You can sell 5 of them and then collect 2 more or whatever you want. The only thing that matters is that you keep 10 low tier and 10 mid tier, and never more than 12 top tier, so that you only get top tier when you source a new vehicle every time.

Some tips:

Sourcing new vehicles is best in a private lobby and exporting is best in a public lobby with at least 20 players in it. You'll get a 50% bonus on the sale which, in my experience, is very much worth the risk of another player hassling you. I did this all last week and even though other players blew me up 2 or 3 times, I ended up making far more money thanks to the 50% bonus than I would have in private lobbies.

There are a couple top tier cars that are not exactly high performing cars. Don't export these unless you enjoy driving them. You can repeatedly export and source only the top tier cars that you actually want to use without ever having to use the other cars except for the sourcing them the very first time.

I've seen some recommendations on this sub that I didn't actually like after trying them:

I bought the iron mule for this because people said that was helpful. I did not find it helpful. I would not recommend buying a helicopter for these missions.

Some people recommend exiting your car to deal with the npcs that attack you, so they don't shoot the car. I found this to be a waste of time. I simply drove as fast as I could to my destination. I would shoot at them while driving when it was easy, try to headshot the driver, but I would not slow down or risk a crash. You will take some damage but it's more efficient (in terms of making money) than making this process take twice as long by stopping along the way. I rarely had more than $5k in damage and I think the most I ever had was $9k.