There's only one reason they want to buy your shares, and that's because they're valuable.

If you think the company has good prospects, keep the shares. If you're right, outsiders will eventually agree with you and invest.

If you think the company is not a good investment, ask the co-founders how they're valuing the company for potential investors and ask for the full share value plus a premium.

If they won't pay a premium, you know they're BSing you and they're just hoping you'll be dumb enough to give them your shares cheap in advance of a deal.