So, I got it working for TBC, sorta. I don't know how to add the new items included in TCB (I know exactly zero coding language but I am good at sussing existing code out, so trying to figure that out).

If you want to get at least the classic list working again in TBC, open Main.lua in a program like Notepad++ and go to line 8, and change the line so it looks like this:

local classicOnly = select(4, GetBuildInfo()) < 30000

You'll also want to make this change in line 5 of the Options.lua file, however it has a ; after the 30000. This will enable the addons option tab in Interface to appear again.

This should get the tooltips for Vanilla items working again, if nothing else. I tested this on my own client and I got back that bit of basic functionality. If I can ever figure out anything else with the code I'll report back, but for now at least it's something for those of us leveling through Vanilla content.

If anyone actually understands LUA, one issue I can see just in this is that it doesn't populate zone or coods if there is only one vendor that sells an item; it give it if there are two or more, however.

In reply to ncplus:

 Can someone do a fork or fan update?

@Katlamos - Thanks for doing this much!

 ohh I miss this, Can somebody take it over?

The original Swindler Preventer seems to be working for me in TBC while we wait for an update.

https://www.curseforge.com/wow/addons/swindlerpreventer

(Sometimes throws up errors but still shows me the info in tooltip)

In reply to aisenfaire:

 Hmmm it doesn't seem to be working for me. 

Just started getting a mapDetails error on retail.

I use wowretail and reported this bug:

Message: Interface\AddOns\FleecingTip\Main.lua:129: attempt to call field 'GetDate' (a nil value)
Time: Fri Oct 23 15:17:07 2020
Count: 3
Stack: Interface\AddOns\FleecingTip\Main.lua:129: attempt to call field 'GetDate' (a nil value)
[string "@Interface\AddOns\FleecingTip\Main.lua"]:129: in function `CheckEventActive'
[string "@Interface\AddOns\FleecingTip\Main.lua"]:518: in function <Interface\AddOns\FleecingTip\Main.lua:302>
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: ?
[string "=[C]"]: in function `SetInventoryItem'
[string "@Interface\FrameXML\BankFrame.lua"]:49: in function <Interface\FrameXML\BankFrame.lua:46>

Locals: worldEvent = "Hallow's End"
WEList = <table> {
Harvest Festival = <table> {
}
Midsummer Fire Festival = <table> {
}
Lunar Festival = <table> {
}
Love is in the Air = <table> {
}
Hallow's End = <table> {
}
Noblegarden = <table> {
}
Brewfest = <table> {
}
}
(*temporary) = nil
(*temporary) = "attempt to call field 'GetDate' (a nil value)"
CALENDAR_FILTER_DARKMOON = "Feira de Negraluna"
UNKNOWN = "Desconhecido"

Fixed it. the C_Calendar.GetDate function has been changed to C_DateAndTime.GetCurrentCalendarTime ... yay ... (hear the sarcasm? lol)

Lines 97 and 129 need to be changed to 

C_DateAndTime.GetCurrentCalendarTime

 so they will look like

line 97

local date = C_DateAndTime.GetCurrentCalendarTime();

and line 129

local calendar = C_DateAndTime.GetCurrentCalendarTime();

In reply to dredd65:

 Thanks for the update. Realized after I posted that there was an update. Cool addon!