There's a radio advert for Charles Hurst Motors (near Belfast) which says (if I recall) that they take trade-ins, and as the ad also mentions £-€ rate, doing all the paperwork, etc, I guess it's being targeted at car buyers down here!
Of course if you're buying a car cheaper up north, your trade-in will be worth less than it might be down here too.
I have had a poor experience with the above garage. I was interested in a car they had to sell and I asked would they take my southern registered car. After giving them the details they indicated that they would make a reasonable offer on my trade in. They indicated they would need to see it to make the final offer [which all made sense].
Bearing this in mind and that the car I wanted was a good price with higher spec [compared to in south!] I travelled the five hours to Belfast, prepared to take a reasonable offer for my souther reg car. Was greeted nicely by manager I had spoken to.
This is when thinks started going bad. Was passed of to a sales rep who showed car, test drive etc. He then asked to see my car and dissappeared for ten mins after which he came back and indicated he would not make me any offer on my trade in! He said he spoke to their agents in the south who they pass the cars on to and they did not want the type of car. This annoyed me as they could have simply made this phone call prior to me travelling for five hours. On top of this the sales rep was ignorant to knowledge of the car market in the south. When I indicated the price range I was looking for in € [as they indicated the car would be going back to the south] he indicated this was way off compared to the North - But any one with a brain knows that anyway.
So a word of warning - take anything they say on the phone in relation to making you an offer with a pinch of salt. In reality they are sending the car back to the south and as the markets stagnant here - well I will say no more.
If its only a short journey then it may be worth the risk. However if your planning to travel a bit make sure they have contacted their agents in the south to get a commitment on what you are trading.
Mostly it wouldn't make sense for garages in the North to take trade ins - you would be better to sell privately down here. Check out Ulstertrader.com for a guide to importing a used car from Northern Ireland though - even eith VRT the savings are huge.
Unless your car is less than 4 years old, most garages will sell on your trade-in at a trade price to a dealer who targets the lower end of the market.
The price they will get is similar to the price anyone would get at an auction. - Maybe a little more.
If you're buying a new car at 20k you will get approx. €2000 more for your trade in than if you sold it through an auction. But likewise if you walk into a garage with no trade in, you should get approx. 10% off the retail price. - Up to 15% on 2nd hand cars if you're good at haggling.
This applies in virtually every motor market worldwide.