4
Just because it uses GNU/GPL software doesn't mean that it has to be sold for nothing.
If you have built something that someone else wants to pay for, you can sell it. No problem there.
I could even sell the module, I released earlier today. Nothing in the GNU/GPL prevents me from charging $100 for granting people a downloadable copy. However, I cannot restrict what the buyer does with it - so they can put it up for free download and I can do nothing about it.
I can also tell people that I can sell them a website even if all I'll be doing is send them a copy of XOOPS with a theme and modules that I download from xoops.org - that would not breach any license either. It would be rather stupid of them to pay for what they can download, but it would not be against any rules.
If you have a website that you can get (enough) money for, then what they are buying is:
1) The domain
2) The concept
3) Your work in setting it up
4) Your visitors
And that should be worth something. If it is worth more than the company is willing to pay, you should keep it - but if the offer is good enough, I'd say go for it. (This obviously depends on how much time, effort and love, you have put into the site, concept etc. and how things are progressing - I mean, the YouTube guys probably loved their site and watching it grow, but seriously, enough money can buy a lot of love
)
If it all goes haywire after the buy, you can always build a new site, the way _you_ want it done
Anyway, it's your choice, XOOPS will love you either way.
"When you can flatten entire cities at a whim, a tendency towards quiet reflection and seeing-things-from-the-other-fellow's-point-of-view is seldom necessary."
Cusix Software