Your assumption has several errors.
If you take an existing website on example.com and move it 100% to a new domain, say example.eu, retaining the exact same url structure and then 301 example.com to example.eu, within a few weeks GooYaBing will have re-spidered and will show the new urls with hardly missing a heartbeat.
I have done this several times without any problems nor loss in traffic and have re-used the orignal example.com for something else entirely within 3 months.
So:
1) A new domain name is by default sandboxed
Not in my experience, I've never had that in 18 years developing.
2) It takes years (2 at least) to start ranking with same pages
I already have in a new domain
Read my explanation above. From where did you get the 2 years idea? I can construct new sites on new domains and have them start ranking within weeks of being launched. For instance I have a 500 page site launched less than a year ago that ranks #1 for most keyword products and ALL keyword products are top 10.
3) The links I got for my domain will be lost (it's a real pain to make all those change to the new one)
This is the biggest pain IF you have a lot of backlinks. Initially when you launch example.eu and point example.com at it you WILL retain all your backlinks since nothing will have changed however when you transfer example.com those backlinks will disappear.
Therefore the questions are how many backlinks do you have and do they really send much traffic?
4) I'll be loosing Adsense income and can't really trust in I will recover that
Your existing AdSense income will more or less stay the same until example.com is transferred the question goes back to #3, realistically how many people click thru to your site from the backlinks.
5) I'll have to advise each costumer the new contact data
Not a problem if you already know all your customer contact details surely?
#6 You have not mentioned...think about all your passwords and YOUR important contact details without which you may not even be able to do many things.
$10,000 is not a lot for a domain, I had one name and the buyer initially bid $10K. When I explained to them the importance of the name to me, the many years of marketing used by the name and the amount of websites I would have to alter if I were to sell it, they slowly increased their bids to $100,000.
The more I analysed the volume of work involved with selling that domain, and all the things I would have to do, I actually told the buyer not to bother any more since it was going to take a least two or three people up to 6 months work and was something I was not prepared to go through.
It really will depend how big your site is and how much $10k is worth to you versus $5k from AdSense and moving to a new domain and the possible loss of traffic from your existing backlinks.
I feel the easy way forward for you is to check WMT if you it them to see how many backlink referrals you get or if you have a CPanel then using Awstats to check how many referral sites you have...you may be surprised how few!
