AITAH for haggling someone down only after I got their heavy furniture into my car?

The other day I found a good deal on an antique dresser for my new apartment. It was listed on fb marketplace for $800 and I thought that was a fair price but also thought I could get them to go down a little.

I arrived at their apartment ready to haggle, but first I wanted to see it and make sure everything checked out. They lived on the eight floor and there was no elevator so that was a bit annoying considering I used to have asthma and they did not have a disclaimer about the stairs on their post. So I was a bit pissed. But the dresser was as advertised and the woman was nice so I held my tongue.

She gave me a weird look when I asked for help getting the dresser down, which was like… WTF? You didn’t tell me you lived on the eighth floor of a stair dependent complex and you expect me to carry this by myself?

So I got her to help me get the dresser down to my car and shove it into the back. I was almost ready to drive off when she reminded me I hadn’t paid her. Moreover, I realized I hadn’t had an opportunity to haggle like I planned. I decided my bargaining power was quite strong since I already had the dresser in my car, so I asked if she would be okay with $450. She didn’t look to happy and there was a bit of back and forth, but eventually she came to see that if she didn’t acquiesce it would be on her to get it back up alone. So she accepted the offer and I took it home. I would have felt bad if not for the false advertisement about stairs which could have caused me an asthma attack.

Well now her husband is messaging me on fb saying I need to pay full price, that what I did was predatory. I haven’t answered yet and not sure I even should. What do y’all think?