I hate trying to buy from dealers

Looked at an 20k mile 07 z06 yesterday. It’s listed online for 40k. Drove fine, dealer had no record of the heads being done. It’s yellow, not my favorite, but I don’t think I hate it. Went inside to talk numbers, I tried making no mention of paying cash to see if I could get a good deal (early Z, heads haven’t been done, it’s yellow, has been for sale for four months, it’s January) I offered 35k, which I think is fair. They came back and said that the online price was a glitch, it was supposed to be 50k, and any discount they could give me would be like a trade credit. Then they said “we don’t want to waste your time, so we’ll take the 40k but you have to buy it today” I told them I would’ve paid the 40 if they included sales tax/tags and had they not pulled the BS of the price change. This is a Chevy dealer, not a buy here pay here lot, lol. I’ll see if it’s still for sale a month from now. Just seems like every time I go to a dealer they try to screw me over