Technically, I don't think they're doing anything wrong. It's kind of annoying, and possibly not the highest morality in some cases, but it's not actually wrong.
There are probably several factors:
- Vendors worried about upstream price changes
- Vendors unsure of what the market will bear / testing the waters
- Vendors playing favorites with past customers
- Vendors charging people more if they appear to have a lot of money
- Vendors trying to engage people personally to seal the deal
- Other?
On the other hand, if Lud wanted to tell people they had to post prices, that wouldn't be wrong either. Vendors worried about legitimate issues like upstream pricing could always include disclaimers.
One group or the other will just have to suck it up, I guess.