Given that, it might be instructive to get a handle on what the buy values at $50,000. Could just be the domain name and/or the "host authority" (when a lot of other (legitimate) sites refer to your site on the web it is perceived by search engines has having a bit more 'authority' than other sites.

If it's the latter you might find they buy it and replace your content with a bunch of links to porn web sites. You'd want to be ok with that before you sold it.

It's going to be pretty hard to estimate value without some idea of revenue potential. In the end, a valuation is going to come down to expected value: https://en.wikipedia.org/wiki/Discounted_cash_flow.

Apparently the buyer expects the value of your site to be >$50K for them. Question is, what's it worth to you?

Why not build a model of your revenues over the next couple of years in excel and calculate their sum?

Obviously a model can only tell you as much as it can accurately predict and you’ll probably want to generate some best case/worst case scenarios but then you’ll at least have a set of numbers to work from and know whether the offer is high or low compared to what you could make.

If there is no revenue, then the offer is likely a one-off opportunity. Don't expect a bunch of prospective buyers even if you hassle their CorpDev departments or potential decision makers.

$50K is likely a guesstimated amount, not based on some kind of calculations. It's basically "looks like it might be valuable, so let's give him some figure big enough for an independent developer but not too much".

Personally, I would haggle a bit (say, try to raise it $80K, then meet around $60K-$70K) and sell. Do not underestimate the uncertainty and the amount of money that needs to be poured to monetise the business. If it's coming from a small company or a person, they are more flexible.

Sell. Make something else.