FWIW, I have a serious stamp collector friend in US that buys bulk collections and sells parcels off on EBay. When he mails out the sales, he uses early mint stamps as postage as the market value of those stamps is less than face value. He acknowledges that what he does is more a hobby than a profitable business venture.
I had what I thought was a very decent US collection that probably cost me many hundreds of $ to assemble. He sold it for me on EBay for $23.
If you have truly rare stamps, there can be some value in those stamps. The rest of the collection is probably worth little to nothing in US IMHO, even less in TH.
Very large spread between buy and sell prices, you would probably receive wholesale price at best.
PM me if you want my friend's contact info. He could probably give you an opinion of value.