Just on the off-chance that you missed the existing option in-game, you can set one of your inventory bags as your sell-bag so that vendors will only offer to buy from that bag (saves you having to search through all of your items).
Open your bags; for each one, right-click and choose "Bag Options" and for all but the sell bag check "Contents are not for sale".
If you only keep vendor items in your sell-bag, when you visit the vendor you can simply spam the "sell" button until the bag runs out of items.
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