It's fine per the current forum rules (note: the up to date rules were only posted in English so far). The old rules disallowed selling stuff on the forums, but tolerated calculator-related sales discussion.
The only kind of rule there can be is that Omni is not responsible if any form of calc purchase/sale/transaction done between forum members goes wrong or something. Or if someone registers only to advertise his own calc site, random products that haven't anything to do with calcs (such as casinos or pharmacy), or if an account is a spambot.