96 jetta GL

So I bought this car yesterday has about 147 K on it a little bit on the high side but the place where I got it does in house with no finance charges. test drove it drove good has a sun roof I was stoked. Guess I didn' test drive it far enough, As I was getting on the freeway the trans started slipping had the car in drive doing about 30 the RPM's jumped to 5k once I got the car up to speed it ran fine. Is it safe to say that the trans is shot and needsrepair replace??? or possible adjustment how much ballpark??? Luckly I was able to retur n the car at no cost. I really want this car any help is appreciated. :banghead: