When you refer to the thing as 'it', it normally means something that you were already talking about. So you could say 'That is a beautiful picture. Are you selling it?'. But if it was the start of a conversation you wouldn't say 'it', perhaps point to it and say 'Excuse me, are you selling this?'
True. My issue is more with Duolingo using phrases as sentences anyway (things that always sound like they're said by a child or non-native English speaker. So on that end, I have to pretend the phrases were plucked out of a book where they do make more sense in context.
Tl;dr: You're saying in English it sounds odd as a stand-alone sentence, but it's a legitimate translation otherwise? Like if you walked in at the tail-end of the conversation, you wouldn't think - that's s weird thing to say!?
Good point about things that sound like a phrase from a non-native English speaker. I just thought that was just the Duolingo Hebrew module - it often seems that the creators are not fully fluent in English.
But about the sentence... I'm not sure which version you are asking about. But assuming it is in the middle of a sentence, 'Excuse me, are you selling it?' sounds fine. Or 'selling this' or even 'selling that'.
And whether it is ok the say 'Sorry' for 'סליחה', might depend on where you are speaking English. In some countries it would sound like an apology, but in other places it would be fine..
If you are using a smart device, you can just change your keyboard settings or keyboard app. If you are on a computer you can change your settings (and imho it's also advisable to buy a silicone keyboard cover in Hebrew or Hebrew/English stickers. If English isn't your native language, they have other options as well. I know someone who got an Arabic/Hebrew sticker pack... They're transparent. They have ones that the sticker language only appears in the bottom corner of each key, so your regular language keyboard is still useable. See 4keyboard.com for examples).
I used the DuoKeyboard extension in Firefox for a while, and it worked pretty well for Hebrew. However, I'd recommend downloading and spending some time learning the keyboard for languages you're learning through your computer's settings. DuoKeyboard sometimes has its own unique layout which won't do you any good off the site, and for some languages there's just nothing at all (e.g., Japanese). It's pretty easy to figure out via Google.