With regard to valuation: Past performance is no guarantee of future performance.
What I am saying is that you need to consider;
- market growth or shrinkage
- vulnerability of the business' model to disruption
- market positioning and any future development done
Escrow.com is ok for simple domain name transfers.
Complex business transfers are usually handled by attorneys.
IMHO, you want to recapture your investment in no more that 2 years. Exception being real estate or certain other hard asset investments.