Typically an e-commerce site will sell for either a multiple of net profit or multiple of revenue. Multiple of profit is common when it is established, relatively stable, and experiencing organic but not rapid growth. Multiple of revenue is more common when it is fast growing and has obvious scale / resource issues (to the point the purchaser knows the margin can be much higher than the current owner is getting). Profit multiples are typically 2x to 3x (and as high as 5x-7x if the business is very fast growing and profitable), and revenue multiples anywhere from 1x to 2x depending on future growth potential (although usually closer to 1x). These would be multiples of trailing twelve months profit or revenue, respectively.
The other things you really want to understand are sources of traffic (ideally it's a relatively even mix of direct, organic, paid, and other sources), strength of email list (not just size but CTR, etc.), size of customer list, % of sales to repeat customers, vendor relationships and if there are supplier contracts can those be assigned and when do they expire, and you really need access to analytics to dig deep on conversion rate, top performing products, etc.
So, do your diligence. Also consider it may not need to be cash upfront - think about proposing an earn out to the seller, or asking them to carry financing (such financing secured only by the assets of the business and not personally).
Anyway if you're looking at a business in this price range you've probably already thought of most of this stuff.
