I believe 97 up has a beefier transmission as well as the more powerful engine. 97 has a one year only lower control arm that is impossible to get. The radio and HVAC units were redesigned in 97 (I believe it was 97).
In the last few months a 50k Riviera with bad paint might have pulled 10k. In mint condition, 10k would have been a lot more likely. If it's true that used car prices are dropping off now, 6k would be reasonable IMO.
8th gens are getting rarer and rarer. ATM there is only one for sale on eBay, a clean 99 with 115,000 miles asking 12,500. I don't believe it will go for 12,500, but you can understand how someone with a 50k car would see high asking prices on eBay and believe they'll get 10k.