Woah, I genuinely feel for you OP. This would really piss me off. All respect to your mum and all, however, I do feel its a little bit low of your mother to be doing this.
I do find it odd that she told you about it-- even more to the point, the fact that she is okay doing it, in full knowledge you also know.
No offence and all, and I'm not trying to rattle you up. Are your family particularly religious? I get that impression from the under tones of the thread. In that case, I feel its also wrong, as a Christian should not conduct themselves in this manner. Not that I am a devout Christian myself, I just found it rather bizarre.
Also, fluttering your eyelids at someone could've been harmless flirting, and although its hard to accept this, it happens all the time. I know its not really comfortable to know as her son. The other thing about have sex with the guy that fixes your house is something else entirely. As is her saying your not 'doing too bad out of it'. Thats almost like your mother validating herself for it, because she's benefiting from it, and you're somehow a beneficiary of it.
Its not exactly prostitution per se, but it is a very transactional way of your mum viewing her sexuality. Exchanging favours for perks and advantages.
I don't think its very courteous of her to do it, let alone tell you and assume your okay with this sort of behaviour- no matter what your age.
I'd try and have a word with her and get her to nip it in the bud. It could be that your mum has low self esteem or this is a projection of how she feels. Maybe she feels aged and so wants to exude her sexuality to prove to herself she still 'has it'?
Either way, she has to respect your wishes and you need to communicate how you feel or you could move out.
Are you sure she wasn't trying to tease you about it- jokingly?