is it a consulting firm who is offering to buy it?
Why not give them a license for the current iteration for the price they want - inclusive of "N" updates - but they have the right to white label the service to their own needs.
In negotiations, assuming they offered you 100K cash - Tell them you would OEM for 75K cash and an annual support of 25K which would include some level of support. The support fee you could use to hire an intern to manage their support questions.
Iterate the site out -- have a vertical for .txt, .html, .JS etc...
Allow file tags, search, families, etc.. etc...
Put together a quick roadmap of these ALREADY planned features to get them to agree to the OEM deal, knowing that they will get the verticals as they come -- but that each vertical will have the 25K annual support license.
So, for the 1st iteration of 12 months of .txt - they pay 100K - next year they get .js, .py and a renewal for .txt and have to pay another 75K
Allow for them to submit update requests which you will value and include for $N... This will allow them a feedback loop and you an income.
If they say no -- hold out and IMMEDIATELY add one of these features, as they will be watching and say "shit - he is doing all that roadmap without us -- fuck it lets bite"