Do merchants expire, or do they share loot tables?

[Rex is back, baby!]

I ask, as a completionist, and if I have a LOT of spare gold, should I buy unique items from them sooner rather than later?

If I progress to a new act, will I forever miss out on that item they carry?

Thank you in advance o7

[Jiroa]

No, they add new items as the game moves on by the old stock stays.

[76561198095755556]

There is some merchants with unique items. If you dont buy them and continue with your playthrough then there will be no other possibility to access exept for mods. The most important is merchants from act 4: all traders from flesh market and storyteller. The last is pretty dumb btw. For whatever reason he will not sell anything after act 4 which makes no sense.

[Yannir]

It's actually a bit complicated. Some stocks are available in many stages of the game and get updated every now and then. Some are always the same and limited availability. Items DO NOT get removed from stocks but the merchants themselves might only be available for a single Act.

THIS GETS SLIGHTLY SPOILERY FROM NOW ON SO READ ON AT YOUR OWN PERIL.

This pertains to Act 4 in particular that has a bunch of vendors only available in that Act.

Most notable merchants:
Woljif - His thiefling stock is always accessible, barring a short period in the middle, and gets new items added every Act.
The "scroll merchant" - Rathimus initially but his stock moves to different characters later.

Those are the only 2 merchants that you will have access to in Act 6.

Some others you have repeated access to:
Wilcer Garms - Available in Acts 2, 3 and 5.
Innkeeper - Unavailable in Acts 2 and 6. IIRC Storyteller has this stock in Act 4.
Drezen has a few other merchants that are available in Acts 3 and 5.

Skeletal Merchant - Available in Acts 3 and 5 but only randomly when moving around the world map. I'm a bit hazy if he shows up in Act 2? I got really unlucky with this one on my Azata pt. Only saw him once in Act 3 and he had the wrong stock on for what I was hoping to buy. Then never saw him again.

Other merchants are bound to locations. So if the location becomes unavailable, so does the merchant most often. Act 1 vendors follow you through the game but are sometimes unavailable. Prologue vendor you'll never see again.