2007 Mazda5 Manual Transmission - Works fine when cold, then becomes stiff and hard to shift after driving

I'd like to get some input into this issue. 2007 Mazda5, manual transmission. Approximately 130k miles. The stick is very easy to get into and out of gear when the car is cold. Then, after driving for a while, the shifter becomes stiff, and is very difficult to get into and out of 5th gear especially. I really have to pull on it to get it out of 5th.

I have had this car for about 4 years. It's never bothered me enough to get it looked at, but now I'm trying to sell it. I took it to a local transmission shop. They said they disconnected the shift linkages and that doesn't seem to be the problem, so it must be internal damage. That doesn't seem plausible since it shifts fine. Even when I have to force it into a gear, there is no grinding or anything. It's never not-engaged, if that makes sense.

Given there aren't any replacement transmissions around anyway, I'd like to see if anyone else has had a similar problem, with fingers crossed there's an easy and/or cheap solution.