Are your websites earning any income? The sale of any website is almost 100% involved with the past, present and future income of the website. Other factors come into play such as how easy it is to maintain the website, does it sell anything etc. But the number one factor is how much money does it earn.

One question is whether you should sell at all. Websites sell at fairly low profit multiples, so a website that generates an income may be better treated as a cash cow: stop putting work into it and let it run down, but monetise heavily.

I need the most basic characteristic that have to have a websites in order to sell it.

This is very general and it depends on the interest of your clients and the potential they see on your sites. You (we...) might see no value on certain websites matching no points of our lists... while others would still be interested.

My two cents on this:
- make clear to your client that your website is not penalized
- content: make your best to proove your website is an asset and not a potential problem (scrapped content)
- Adsense friendly (not in danger, banned or whatever)

and safety... search for services to sell and buy sites. Many things might go wrong when you do it (giving away usernames and passwords) and some have lost their websites or faced akward moments while solving a confusing switch.