Extra hard question about GST

Setup is pretty straightforward. Limited tiny company, GST registered, Xero for accounting,
The main business idea is buying and selling retro/vintage goods, with some mild restoration work. Some of the items will be soursed localy (like flee, FB market), some online via TradeMe, some oversea.

There are few questions about tax applications, so let's begin
1. How to comply for GST 3/23 credit on second-hand goods bought from TradeMe? IRD said that you need date, price, count, seller name, address... everything is in place, but TradeMe is not providing sellers address, so technicaly I can't comply? Asking seller for his home adress doesnt feel right, especialy if dealing with $5 item.
2. Fair Market Value issue. For accounting purpose, sometimes I need to find out Fair Market Value(FMV) for item, that never have been sold publicly in NZ. On the other side, there is Ebay, and some oversea resources who calculates, tracks and provides wordwide FMV. Can I use this information for IRD? Also sometimes item can be undervalued in NZ, like sold here on multiple occasions for $10 and it's real market value on Ebay is $200. What FMV I should apply here?

I spent days trying to google answers online and in various IRD documents, so any help would be great!