You can make an offer on a house contingent on the sale of your home. The seller would likely negotiate a price with you and still keep the house on the market but give you first right of refusal if another offer came in.
It doesn't cost you anything to use a realtor - find one realtor that you trust and they will keep you informed of anything that comes on the market and the realtor will represent you. The realtor has a fiduciary responsibility to look out for your interest. If the same realtor is representing the seller also, they are having to walk a tight rope between the 2 of you. It can work to your advantage if the realtor is willling to take a discount on the commission, but it can also result in problems if you don't feel like the realtor is looking after your best interest as well as the seller's. FSBO can save you money, but in most cases the seller feels like he has a right to save the extra money which doesn't result in any advantage to you. Also, many FSBO get very emotional when pricing their homes and feel the house is worth more than the market supports. The majority of FSBOs I see are overpriced.
It is certainly in your best interest to have 20% down if possible and avoid PMI altogether. I agree that too many people are getting into home ownership without enough vested interest in the house - I used to do mortgages for a commercial bank back in the late 70's and early 80's. I never made a mortgage to anyone with less than 30% down. That was also when interest rates were at 15%+. I really don't get that involved with the financing end any more but if I can answer any questions for you on the buying or selling end, just let me know.