Hi immanuelbaan
Thank you for your post! 
When you have stock management there actually is a function to do this already. When you make a credit note on the returned obsolete goods - you can on each product line edit - and then mark the box "obsolete".
I think this meets your need - let me know if it doesn't.
<p>
Med venlig hilsen</p>
<p>
<strong>Peter Luthman</strong></p>
<p>
<strong><img alt="" src="
style="width: 200px; height: 49px; " /></strong></p>
<p>
<a href="http://www.e-conomic.dk/"><strong>Online regnskabsprogram</strong></a><br />
</p>
<ul>
<li>
<a href="http://www.e-conomic.dk/support">e-conomic support</a> - Din vej til vores support</li>
<li>
<a href="http://wiki.e-conomic.dk/">e-copedia</a> - Opslagsværk om e-conomic</li>
<li>
<a href="http://blog.e-conomic.dk/">e-conomic blog</a> - Hold dig opdateret</li>
</ul>