The trans is $3500 in these parts. If he agreed to $5K for a good truck, that same truck less a trans is worth less than $1500.
But it sounds like it has a bad miss, not a bad transmission. Let me guess that the plugs were never changed and with a set of plugs it will be fine. If the problem is the transmission, changing the fluid won't do anything, except of course ruin the transmission if the job's done wrong or the wrong fluid's used.
I'd tell him the change the plugs, don't touch the trans and if that doesn't solve the problem, I'd walk out of there.