I can't comment on your lag issues, I haven't played the recent patches, but for merchant prices the character who interacted with the merchant will use their CHA to modify buy/sell value. It's a horrible system, forcing you to painstakingly move everything you want to sell to a single character. There are also old D:OS2 features behind the scenes, for example you can just gift a merchant some money which will increase their favor of you and lower all their prices. Right now that isn't very surfaced and I hope they just remove it entirely. As for the value vs the buy/sell price, it's pretty common in games for it to be like this one: merchant buys for less and sells for more (shrug).
I expect them to sell for more than they buy it for - that's to be expected. I am not intimately familiar with 5.0, because it is just poorly designed from what I am aware of, but the standard before that has always been that they pay half price on everything but trade goods. I expected half price - I sell for half the value but only pay the value. That has a factor of only 2 - something much more reasonable than 5.75.
