Try to frame it in terms of $/user/mo over the next n months.

So $50k up front is ~$4K/mo for 1 year (or $5/user/month).

If you monetize, what would be your expected $/user/month? Your buyer certainly has some figure in mind already, and they're no-doubt low-balling you.

Is user growth rate positive or negative? What percentage growth have you seen MoM or YoY?

I would spend some time modeling the various scenarios to put the $50K into perspective. For example, if you plan on running this site for the next 5 years and you make $x/user/month and you have positive user growth (say 1% net user growth each year), etc.