This one has me a bit stumped; I have been playing Skyrim SE for about 70 hours with a couple of mods and for the last hour, every time I talk to a merchant and ask what they have for sale, they say "have a look" or whatever they normally say but no menu or inventory comes up so I cannot sell or buy anything from anyone. I had to invoke a cheat to carry my stuff and try different merchants in different cities but all the same. I did try disabling the mods but it made no difference.

Current Mods [in order]

Unofficial Skyrim SE patch

Hearthfire Multikid

Horse storage

better hearthstone mat

Whistle

Transfer you gear

Relationship dialog overhaul

Seraphim female body

Sofia

True storms

Realistic conversations

Big: more kill moves

Skyrim player revoiced

Not sure what else to try [the last 4 mods and the patch were added after the glitch started, I thought the patch and realistic conversations might kick start the merchants back into working again].

Try uninstalling the "transfer your gear" mod and making a new character to see if it fixes it.