If they were looking to market a direct sequel to Super Mario RPG, which, as mentioned previously, wouldn't happen because Paper Mario exists, and so does the Mario & Luigi franchise, and Nintendo wouldn't want to steal the thunder from those franchises, because they sell very well, which I'll get into, also, getting Square back in to develop just wouldn't happen. Nintendo is too protective of it's properties now, although, you could argue that's becoming less true because of Hyrule Warriors being a thing, and the fact that Smash 4 is being developed by Namco Bandai, but I digress.
But, if it did happen. I feel Nintendo would not have that hard of a time selling it. Considering the cult status that SMRPG has achieved over the years, and also considering sales figures on Paper Mario: Sticker Star (1.97 million copies as of March 31, 2013), and on Mario & Luigi: Dream Team (2 million copies as of December 31, 2013). The only stop gap where there might be issues, is convincing everyone that they will like it. The Mario & Luigi franchise has received very high reviews throughout it's run. With, "Dream Team", receiving very high scores from most all relevant sources. On the other hand, the Paper Mario franchise, which is considered the spiritual successor to SMRPG, and what I imagine is closer to what the hypothetical SMRPG sequel would be like, excluding the first and Thousand Year Door, has been receiving mixed reviews, regardless of the sales numbers.